Implement webos_build_gtest()

:Release Notes:
Search the source code of gtest
Build gtest as a subproject

:Detailed Notes:
Since gtest source code is deployed with a bitbake recipe in WebOS,
as well as in popular desktop linux distributions, it's necessary
to build testing code without knowing the particular paths in the
file system.
Components should call the function webos_build_gtest() at some
point, and link the unit tests agains ${GTEST_LIBRARIES}.

:Testing Performed:
Built libpbnjson successfully in WebOS
Built libpbnjson successfully in Ubuntu/Arch

:QA Notes:

:Issues Addressed:
[GF-11274] Create cmake function to detect source code of gtest

Open-webOS-DCO-1.0-Signed-off-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>

Change-Id: I2f5cff802c97dd7d748804f005caaa9e7c19f69c
Reviewed-on: https://g2g.palm.com/2985
Reviewed-by: Build Verification
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>