gcc-plugins: Detail c-common.h location for GCC 4.6

The c-common.h file moved in stock gcc 4.7, not gcc 4.6. However, most
people building plugins with gcc 4.6 are using the Debian or Ubuntu
version, which includes a patch to move the headers to the 4.7 location.
In case anyone trips over this with a stock gcc 4.6, add a pointer to the
patch used by Debian/Ubuntu.
Signed-off-by: Kees Cook <keescook@chromium.org>

scripts/gcc-plugins/gcc-common.h

@@ -63,6 +63,13 @@
 #endif
 
 #if BUILDING_GCC_VERSION >= 4006
+/*
+ * The c-family headers were moved into a subdirectory in GCC version
+ * 4.7, but most plugin-building users of GCC 4.6 are using the Debian
+ * or Ubuntu package, which has an out-of-tree patch to move this to the
+ * same location as found in 4.7 and later:
+ * https://sources.debian.net/src/gcc-4.6/4.6.3-14/debian/patches/pr45078.diff/
+ */
 #include "c-family/c-common.h"
 #else
 #include "c-common.h"