gcc: Fix build error on gcc 4.7

Specifically, we are trying to fix the following error seen on Hydra:

../../gcc-4.7.3/gcc/gengtype-lex.c:1:21: fatal error: bconfig.h: No such file or directory

The patch is taken from gcc's SVN revision 193691.
Merge pull request #1518 from wizeman/u/gcc_47_bconfig

gcc 4.7 build fix attempt