libgloss: build with -nostdinc

We don't want libgloss building against C library headers that happened
to be installed with the toolchain, so add -nostdinc to the build.  We
still need access to the compiler internal headers, so probe those and
include them via -isystem.  This uses a similar probing style as glibc,
which has used it for over a decade, so it should be safe & stable.

This should prevent any latent bugs due to testing with a toolchain that
is fully configured & installed already.

libgloss/configure

@@ -5104,6 +5104,26 @@ esac
 
 
 
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for compiler search settings" >&5
+$as_echo_n "checking for compiler search settings... " >&6; }
+if ${libc_cv_compiler_isystem+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+    for subdir in include include-fixed; do
+    if dir=`$CC -print-file-name=$subdir`; then :
+        if test -n "$dir"; then :
+          libc_cv_compiler_isystem="$libc_cv_compiler_isystem -isystem $dir"
+
+fi
+
+fi
+  done
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_compiler_isystem" >&5
+$as_echo "$libc_cv_compiler_isystem" >&6; }
+CC="$CC -nostdinc $libc_cv_compiler_isystem"
+
 host_makefile_frag_path=$host_makefile_frag
 
 

libgloss/configure.ac

@@ -336,6 +336,17 @@ m4_include([mcore/acinclude.m4])
 m4_include([mn10300/acinclude.m4])
 m4_include([moxie/acinclude.m4])
 
+AC_CACHE_CHECK([for compiler search settings], libc_cv_compiler_isystem, [dnl
+  for subdir in include include-fixed; do
+    AS_IF([dir=`$CC -print-file-name=$subdir`], [dnl
+      AS_IF([test -n "$dir"], [dnl
+        libc_cv_compiler_isystem="$libc_cv_compiler_isystem -isystem $dir"
+      ])
+    ])
+  done
+])
+CC="$CC -nostdinc $libc_cv_compiler_isystem"
+
 dnl We have to assign the same value to other variables because autoconf
 dnl doesn't provide a mechanism to substitute a replacement keyword with
 dnl arbitrary data or pathnames.