Don't include subproject headers with -I

Use -iquote instead.  This prevents our include paths from messing up
the system headers depended upon by libstdc++.  (The specific problem
was syscall.h in fesvr/, which was interfering with libstdc++'s
dependence on the system's syscall.h for SYS_futex.)

Subproject headers can now be included in the following ways:

    #include "foo.h"      // for a header local to this subproject
    #include <bar/baz.h>" // for a header in another subproject

But no longer:

    #include <baz.h>      // for a header in any subproject

As a special case, libfdt needs itself to be added to the -I path,
because their coding style is to use angle brackets for local headers.

Makefile.in

@@ -69,7 +69,7 @@ install_exes_dir := $(INSTALLDIR)/bin
 sprojs         := @subprojects@
 sprojs_enabled := @subprojects_enabled@
 
-sprojs_include := -I. -I$(src_dir) $(addprefix -I$(src_dir)/, $(sprojs_enabled))
+sprojs_include := -iquote . -I$(src_dir) $(addprefix -iquote $(src_dir)/, $(sprojs_enabled))
 VPATH := $(addprefix $(src_dir)/, $(sprojs_enabled))
 
 #-------------------------------------------------------------------------

fdt/fdt.mk.in

@@ -15,3 +15,5 @@ fdt_c_srcs = \
 	fdt_empty_tree.c \
 	fdt_addresses.c \
 	fdt_overlay.c \
+
+fdt_CFLAGS = -I$(src_dir)/fdt

riscv/riscv.mk.in

@@ -7,7 +7,7 @@ riscv_subproject_deps = \
 	fesvr \
 	softfloat \
 
-riscv_CFLAGS = -fPIC
+riscv_CFLAGS = -fPIC -I$(src_dir)/fdt
 
 riscv_install_shared_lib = yes