mesa: refactor: move #define FEATURE flags into new mfeatures.h file

Also, check the FEATURE flags in many places.
(cherry picked from commit 40d1a40f294f1ed2dacfad6f5498322fc08cc2d1)

Conflicts:

	src/mesa/main/config.h
	src/mesa/main/context.c
	src/mesa/main/texobj.c
	src/mesa/main/texstate.c
	src/mesa/main/texstore.c
diff --git a/src/mesa/main/api_exec.h b/src/mesa/main/api_exec.h
index 7f15ea9..4bd7150 100644
--- a/src/mesa/main/api_exec.h
+++ b/src/mesa/main/api_exec.h
@@ -27,7 +27,10 @@
 #define API_EXEC_H
 
 
-void
+struct _glapi_table;
+
+
+extern void
 _mesa_init_exec_table(struct _glapi_table *exec);