Allow users to build QEMU with system-mode emulation

I'm trying to clean up some of my test scripts, and one issue is that
I've got two QEMU builds: one for user-mode emulation and one for
system-mode emulation.  This adds an autoconf flag that allows me to
avoid the duplication.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Makefile.in

@@ -36,6 +36,7 @@ WITH_TUNE ?= @WITH_TUNE@
 WITH_ISA_SPEC ?= @WITH_ISA_SPEC@
 SYSROOT := $(INSTALL_DIR)/sysroot
 ENABLE_LIBSANITIZER ?= @enable_libsanitizer@
+QEMU_TARGETS ?= @qemu_targets@
 
 SHELL := /bin/sh
 AWK := @GAWK@
@@ -848,7 +849,7 @@ stamps/build-qemu: $(QEMU_SRCDIR) $(QEMU_SRC_GIT)
 	mkdir $(notdir $@)
 	cd $(notdir $@) && $</configure \
 		--prefix=$(INSTALL_DIR) \
-		--target-list=riscv64-linux-user,riscv32-linux-user \
+		--target-list=$(QEMU_TARGETS) \
 		--interp-prefix=$(INSTALL_DIR)/sysroot \
 		--python=python3
 	$(MAKE) -C $(notdir $@)

configure.ac

@@ -257,4 +257,15 @@ AS_IF([test "x$enable_libsanitizer" != xno],
 	[AC_SUBST(enable_libsanitizer, --enable-libsanitizer)],
 	[AC_SUBST(enable_libsanitizer, --disable-libsanitizer)])
 
+AC_ARG_ENABLE(qemu_system,
+	[AS_HELP_STRING([--enable-qemu-system],
+		[Build qemu with system-mode emulation])],
+	[],
+	[enable_qemu_system=no]
+	)
+
+AS_IF([test "x$enable_qemu_system" != xno],
+	[AC_SUBST(qemu_targets, [riscv64-linux-user,riscv32-linux-user,riscv64-softmmu,riscv32-softmmu])],
+	[AC_SUBST(qemu_targets, [riscv64-linux-user,riscv32-linux-user])])
+
 AC_OUTPUT