gdb version string: ignore (...) content when using rubenvb's build

The often used rubenvb mingw-w64 build reports a GDB version string with
the GCC version in parentheses:
  GNU gdb (rubenvb-4.7.2-release) 7.5.50.20120920-cvs
With his patch the content within the parentheses is ignored, and is not
wrongly interpreted as GDB version.

Change-Id: I1a3c54acc81cb6d649d11ebf38dea96fc2685aa1
Reviewed-by: hjk <hjk121@nokiamail.com>