Some platforms' yval.h defines restrict as a macro when using clang, which causes this file to not compile.

Now use gflags::gflags instead of gflags

note: seems already in use in gRPC
cf https://github.com/grpc/grpc/blob/master/cmake/gflags.cmake#L30

cf https://gitlab.kitware.com/cmake/cmake/blob/master/Modules/CTest.cmake#L88

repair wrong namespace problem

This change avoids having to configure a private .h.in file during the Bazel build such that these files are not available to dependent projects in the GENDIR of the external gflags project.

Note: Leaves behind empty directories, no real uninstaller. Just for convenience of some.

Warnings will spit out on newer compilers due to PRI* being redefined.
Tested with VC++ 2015 and 2017. [Blog post](https://blogs.msdn.microsoft.com/vcblog/2013/07/19/c99-library-support-in-visual-studio-2013/) describing the defines being added in 2013.

The argument-related variables in a macro body are not real variables,
but special substitutions. They cannot be directly referred to by name,
only expanded.

Fix #211