Handle gcc version 5.x.y.

Recognize the future versions of gcc will be called 5.x.y.
Ignore line directives in glibc version configure test (newer
gcc/cpp will mark system macros with an extra line comment).

Patch from  Dmitry Djachenko (dimhen@gmail.com). Fixes #338932.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14507 a5019735-40e9-0310-863c-91ae7b9d1cf9