[config] Don't compile disabled features

This makes it possible to include all .cc files into build, even if not
building CoreText, Uniscribe, etc.

This was mostly to help custom builders.  But also means that we can
include all files in our own build system.  Not sure if we should.
Definitely simplifies things, but slightly only.

src/hb-graphite2.cc

diff --git a/src/hb-graphite2.cc b/src/hb-graphite2.cc
index f799f8b..9588fa4 100644
--- a/src/hb-graphite2.cc
+++ b/src/hb-graphite2.cc
@@ -26,6 +26,10 @@
  * Google Author(s): Behdad Esfahbod
  */
 
+#include "hb.hh"
+
+#ifdef HAVE_GRAPHITE2
+
 #include "hb-shaper-impl.hh"
 
 #include "hb-graphite2.h"
@@ -447,3 +451,6 @@
 
   return true;
 }
+
+
+#endif