configure: Not all compilers understand -idirafter, use -I instead.

If this is a problem, we can write a test for the compiler flag, but shouldn't
we _always_ use our Khronos headers instead of depending on the system...?

configure.ac

@@ -70,7 +70,7 @@ case "$host" in
 esac
 
 dnl Set up the compiler and linker flags
-INCLUDE="-I$srcdir/include -idirafter $srcdir/src/video/khronos"
+INCLUDE="-I$srcdir/include -I$srcdir/src/video/khronos"
 if test x$srcdir != x.; then
     INCLUDE="-Iinclude $INCLUDE"
 elif test -d .hg; then