Generate a libgtest.la to help libtool managing dependencies

When building and using googletest from an install tree
we want libtool to generate rpath in the dependent binaries
to allow them to be executed from a build without a need
for LD_LIBRARY_PATH or similar.

For libtool based project this happens automatically
as projects generates .la files and install them.
Provide such a file for gtest as well to allow it to
work smoothly with libtool based projects.

Signed-off-by: Knut Omang <knut.omang@oracle.com>