This will introduce RAPIDJSON_BUILD_THIRDPARTY_GTEST option. If it is set to
TRUE, cmake will look for GTest installation in `thirdparty/gtest` before
looking in other places.

Current default value (OFF) for RAPIDJSON_BUILD_THIRDPARTY_GTEST represents
previous behaviour when system-wide gtest installation is used whenever
possible.

This commit will as well eliminate problem described in #309 when source
directory found is `thirdparty/gtest` while include files are found
system-wide. This however won't give the user possibility to select gtest
installation to use.