configure: Do a real check for -idirafter anyhow.

(The CMake project cheats around this by asking "are we GCC or Clang?" and I'm
inclined to leave it like that for now.)

configure

@@ -16000,7 +16000,37 @@ case "$host" in
         ;;
 esac
 
-INCLUDE="-I$srcdir/include -I$srcdir/src/video/khronos"
+save_CFLAGS="$CFLAGS"
+have_idirafter="no"
+idirafter="-I"
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for -idirafter option" >&5
+$as_echo_n "checking for -idirafter option... " >&6; }
+CFLAGS="$save_CFLAGS -idirafter src"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+
+int
+main ()
+{
+
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+
+have_idirafter="yes"
+idirafter="-idirafter"
+
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $have_idirafter" >&5
+$as_echo "$have_idirafter" >&6; }
+CFLAGS="$save_CFLAGS"
+
+INCLUDE="-I$srcdir/include $idirafter $srcdir/src/video/khronos"
 if test x$srcdir != x.; then
     INCLUDE="-Iinclude $INCLUDE"
 elif test -d .hg; then

configure.ac

@@ -69,8 +69,23 @@ case "$host" in
         ;;
 esac
 
+dnl see if -idirafter is available.
+save_CFLAGS="$CFLAGS"
+have_idirafter="no"
+idirafter="-I"
+AC_MSG_CHECKING(for -idirafter option)
+CFLAGS="$save_CFLAGS -idirafter src"
+AC_TRY_COMPILE([
+],[
+],[
+have_idirafter="yes"
+idirafter="-idirafter"
+])
+AC_MSG_RESULT($have_idirafter)
+CFLAGS="$save_CFLAGS"
+
 dnl Set up the compiler and linker flags
-INCLUDE="-I$srcdir/include -I$srcdir/src/video/khronos"
+INCLUDE="-I$srcdir/include $idirafter $srcdir/src/video/khronos"
 if test x$srcdir != x.; then
     INCLUDE="-Iinclude $INCLUDE"
 elif test -d .hg; then