Enable precompiled headers when using clang

Makefile.in

@@ -38,6 +38,7 @@ scripts_dir  := $(src_dir)/scripts
 
 HAVE_INT128 := @HAVE_INT128@
 HAVE_DLOPEN := @HAVE_DLOPEN@
+HAVE_CLANG_PCH := @HAVE_CLANG_PCH@
 
 # If the version information is not in the configure script, then we
 # assume that we are in a working directory. We use the vcs-version.sh
@@ -219,9 +220,8 @@ $(2)_deps += $$(patsubst %.o, %.d, $$($(2)_c_objs))
 $(2)_deps += $$(patsubst %.h, %.h.d, $$($(2)_precompiled_hdrs))
 $$($(2)_pch) : %.h.gch : %.h
 	$(COMPILE) -x c++-header $$< -o $$@
-# If using clang, don't depend (and thus don't build) precompiled headers
-$$($(2)_objs) : %.o : %.cc $$($(2)_gen_hdrs) $(if $(filter-out clang,$(CC)),$$($(2)_pch))
-	$(COMPILE) $$($(2)_CFLAGS) -c $$<
+$$($(2)_objs) : %.o : %.cc $$($(2)_gen_hdrs) $$($(2)_pch)
+	$(COMPILE) $(if $(HAVE_CLANG_PCH), $$(if $$($(2)_pch), -include-pch $$($(2)_pch))) $$($(2)_CFLAGS) -c $$<
 $$($(2)_c_objs) : %.o : %.c $$($(2)_gen_hdrs)
 	$(COMPILE_C) $$($(2)_CFLAGS) -c $$<
 

ax_check_compile_flag.m4

@@ -0,0 +1,53 @@
+# ===========================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_CHECK_COMPILE_FLAG(FLAG, [ACTION-SUCCESS], [ACTION-FAILURE], [EXTRA-FLAGS], [INPUT])
+#
+# DESCRIPTION
+#
+#   Check whether the given FLAG works with the current language's compiler
+#   or gives an error.  (Warnings, however, are ignored)
+#
+#   ACTION-SUCCESS/ACTION-FAILURE are shell commands to execute on
+#   success/failure.
+#
+#   If EXTRA-FLAGS is defined, it is added to the current language's default
+#   flags (e.g. CFLAGS) when the check is done.  The check is thus made with
+#   the flags: "CFLAGS EXTRA-FLAGS FLAG".  This can for example be used to
+#   force the compiler to issue an error when a bad flag is given.
+#
+#   INPUT gives an alternative input source to AC_COMPILE_IFELSE.
+#
+#   NOTE: Implementation based on AX_CFLAGS_GCC_OPTION. Please keep this
+#   macro in sync with AX_CHECK_{PREPROC,LINK}_FLAG.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Guido U. Draheim <guidod@gmx.de>
+#   Copyright (c) 2011 Maarten Bosmans <mkbosmans@gmail.com>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
+
+#serial 6
+
+AC_DEFUN([AX_CHECK_COMPILE_FLAG],
+[AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF
+AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
+AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
+  ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
+  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1"
+  AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],
+    [AS_VAR_SET(CACHEVAR,[yes])],
+    [AS_VAR_SET(CACHEVAR,[no])])
+  _AC_LANG_PREFIX[]FLAGS=$ax_check_save_flags])
+AS_VAR_IF(CACHEVAR,yes,
+  [m4_default([$2], :)],
+  [m4_default([$3], :)])
+AS_VAR_POPDEF([CACHEVAR])dnl
+])dnl AX_CHECK_COMPILE_FLAGS

configure

@@ -627,6 +627,7 @@ LIBOBJS
 subprojects_enabled
 subprojects
 HAVE_DLOPEN
+HAVE_CLANG_PCH
 HAVE_INT128
 INSTALL_DATA
 INSTALL_SCRIPT
@@ -2333,6 +2334,45 @@ case $host_os in *\ *) host_os=`echo "$host_os" | sed 's/ /-/g'`;; esac
 #serial 8
 
 
+# ===========================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_CHECK_COMPILE_FLAG(FLAG, [ACTION-SUCCESS], [ACTION-FAILURE], [EXTRA-FLAGS], [INPUT])
+#
+# DESCRIPTION
+#
+#   Check whether the given FLAG works with the current language's compiler
+#   or gives an error.  (Warnings, however, are ignored)
+#
+#   ACTION-SUCCESS/ACTION-FAILURE are shell commands to execute on
+#   success/failure.
+#
+#   If EXTRA-FLAGS is defined, it is added to the current language's default
+#   flags (e.g. CFLAGS) when the check is done.  The check is thus made with
+#   the flags: "CFLAGS EXTRA-FLAGS FLAG".  This can for example be used to
+#   force the compiler to issue an error when a bad flag is given.
+#
+#   INPUT gives an alternative input source to AC_COMPILE_IFELSE.
+#
+#   NOTE: Implementation based on AX_CFLAGS_GCC_OPTION. Please keep this
+#   macro in sync with AX_CHECK_{PREPROC,LINK}_FLAG.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Guido U. Draheim <guidod@gmx.de>
+#   Copyright (c) 2011 Maarten Bosmans <mkbosmans@gmail.com>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
+
+#serial 6
+
+
 # ===========================================================================
 #    https://www.gnu.org/software/autoconf-archive/ax_check_link_flag.html
 # ===========================================================================
@@ -4647,6 +4687,43 @@ fi
 done
 
 
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether C++ compiler accepts -relocatable-pch" >&5
+$as_echo_n "checking whether C++ compiler accepts -relocatable-pch... " >&6; }
+if ${ax_cv_check_cxxflags___relocatable_pch+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+
+  ax_check_save_flags=$CXXFLAGS
+  CXXFLAGS="$CXXFLAGS  -relocatable-pch"
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  ax_cv_check_cxxflags___relocatable_pch=yes
+else
+  ax_cv_check_cxxflags___relocatable_pch=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  CXXFLAGS=$ax_check_save_flags
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ax_cv_check_cxxflags___relocatable_pch" >&5
+$as_echo "$ax_cv_check_cxxflags___relocatable_pch" >&6; }
+if test "x$ax_cv_check_cxxflags___relocatable_pch" = xyes; then :
+  HAVE_CLANG_PCH=yes
+
+else
+  :
+fi
+
+
 #-------------------------------------------------------------------------
 # MCPPBS subproject list
 #-------------------------------------------------------------------------

configure.ac

@@ -45,6 +45,7 @@ AC_CANONICAL_HOST
 
 m4_include(ax_require_defined.m4)
 m4_include(ax_append_flag.m4)
+m4_include(ax_check_compile_flag.m4)
 m4_include(ax_check_link_flag.m4)
 m4_include(ax_append_link_flags.m4)
 
@@ -89,6 +90,8 @@ AC_CHECK_TYPE([__int128_t], AC_SUBST([HAVE_INT128],[yes]))
 
 AX_APPEND_LINK_FLAGS([-Wl,--export-dynamic])
 
+AX_CHECK_COMPILE_FLAG([-relocatable-pch], AC_SUBST([HAVE_CLANG_PCH],[yes]))
+
 #-------------------------------------------------------------------------
 # MCPPBS subproject list
 #-------------------------------------------------------------------------