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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.