This commit addresses a few issues:

    1.  No longer leak config.h in a way similar to
            https://github.com/gflags/gflags/issues/233
        The solution of prefixing the path by 'glog_internal' is modified from
            https://github.com/gflags/gflags/issues/234

    2.  No longer expose internal headers.

    3.  Replace PACKAGE_NAME with native.package_name()

    4.  Uers can choose namespaces via the newly added 'namespace' keyword.

    5.  Replace glob with explicitly listing of files.

    6.  Make the genrules more compact using pythonic list construction.