Fix build with MS VS 2015 RC

Earlier version, such as CMake 2.8.11.2 in particular, have incomplete or faulty support of target_include_directories generator expressions. Moreover, this allows us to assume that the command target_include_directories is available.

The gflags_TARGET variable contains the name of the originally exported import target. The "gflags" import target contains a copy of the relevant properties of this import target created by CMake. The gflags_LIBRARIES variable can be used to be consistent with other packages, but is otherwise no longer needed. In most cases, "target_link_libraries(foo gflags)" should be used.

This avoids having patched CMake modules in the source tree of the gflags library. Usually a C compiler is installed along a C++ compiler (or it is used for C as well). The only slight downside is the extra configuration time needed by CMake to check the C compiler.