Add missing @ to cmake-config.in

Add missing @ to cmake file

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.