Add webos_use_gtest()

:Release Notes:
- Add a new function webos_use_gtest(). It arranges to build the gtest
  libraries as a subproject from their source expected to be found under
  WEBOS_INSTALL_SRCDIR/gtest.
- Set version to 1.0.0 RC5.

:Detailed Notes:
Also:
- Add global constant WEBOS_BINARY_IMPORTED_DIR, which is the tree under
  which built files of imported projects are placed.
- Fix unrelated typo in REFERENCE.md .

:Testing Performed:
- Built libpbnjson successfully in webOS.
- Built libpbnjson successfully in Ubuntu/Arch.

:QA Notes:

:Issues Addressed:
[GF-11274] Add function to cmake-modules-webos to allow projects to use
           gtest

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

Change-Id: Ic64f685680a91e97c83e1e098f17f633d38fff2b
Reviewed-on: https://g2g.palm.com/3185
Reviewed-by: Build Verification
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Tested-by: Keith Derrick <keith.derrick@lge.com>