THRIFT-2894 PHP json serializer skips maps with int/bool keys
Client: PHP
Patch: Stig Bakken <stig@zedge.net>

This closes #320

PHP generator: in jsonSerialize, only skip maps with non-scalar keys (previously skipped maps with non-string keys)
THRIFT-2893 CMake build fails with boost thread or std thread

Following changes are made to fix the build
* Add USE_..._THREAD compiler definitions correctly
* Link to boost_thread and boost_system when configured with boost thread
* Link to pthread if platform is posix and std thread is used
* Use PlatformThreadFactory in test code