Changed namespace for gflags functions to match project

As of [March 20th](https://code.google.com/p/gflags/?redir=1#20_March_2014), the default namespace of gflags (unless compiled with special options) is gflags, not google, which breaks the compilation of flint.

Alternatively, this project could follow the folly project and use the configure script and a #Define to use the correct namespace.  I could try to write a patch for that, if it is preferred.