Use GFLAGS_IS_A_DLL=1 in public headers if BUILD_SHARED_LIBS is enabled (Fixes issue #77).
Add GFLAGS_INCLUDE_DIR CMake variable (Fixes issue #80).

Both GFLAGS_NAMESPACE and GFLAGS_INCLUDE_DIR can be used to customize the installation of the gflags library. One specifies the C++ namespace name of the gflags symbols, the other the include subdirectory path of the public header files. By default, both are set to "gflags", but other Google libraries still expect "google" as C++ namespace name, but not necessarily also include path.