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.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