[Bug libobjc/78697] --with-target-bdw-gc-include shouldn't be required to be per multilib

[doko at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78697

--- Comment #1 from Matthias Klose <doko at gcc dot gnu.org> ---
well, it does make sense when the header files differ, however that is not the
case for the gc header files afaics.  Is this what you are looking for?  The
fallback directory should not override the values given in
--with-target-bdw-gc= from my point of view.

--- configure.ac        (revision 243108)
+++ configure.ac        (working copy)
@@ -256,16 +256,19 @@
       for i in `echo $with_target_bdw_gc_include | tr ',' ' '`; do
         case "$i" in
           *=*) sd=${i%%=*}; d=${i#*=} ;;
-          *) sd=.; d=$i ;;
+          *) sd=.; d=$i; fallback=$i ;;
         esac
         if test "$mldir" = "$sd"; then
           bdw_val=$d
         fi
       done
-      if test "x$bdw_val" = x; then
+      if test "x$bdw_val" = x && test "x$bdw_inc_dir" = x && test "x$fallback"
!= x; then
+        bdw_inc_dir="$fallback"
+      else if test "x$bdw_val" = x; then
         AC_MSG_ERROR([no multilib path ($mldir) found in
--with-target-bdw-gc-include])
+      else
+        bdw_inc_dir="$bdw_val"
       fi
-      bdw_inc_dir="$bdw_val"
     fi
     bdw_val=
     if test "x$with_target_bdw_gc_lib" != x; then