gtest: generate the gtest-config script

gtest has moved from the autotools to CMake, but the CMake build
system is currently not installing the gtest-config script. Work
around that by installing it manually, after doing the necessary
sed-ing from the gtest-config.in template.

Reported-by: Carlos Santos <casantos@datacom.ind.br>
Suggested-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>