A small re-work of the installed m4 script for Google Test. This allows other projects to easily leverage an installation or un-installed build of Google Test from their project's Autoconf script. This re-work specifically introduces the ability to provide a path as an argument to the resulting configure script option which can specify either an installation prefix or a build directory for Google Test.

This change also makes a small portability tweak by using ``s instead
of $() for command expansion.

By Chandler Carruth. Reviewed by Benoit Sigoure.