Conditional and out of source build

* Enable 'make test' target
* Make out-of-source build possible
* Generate samples/monster_generated.h and tests/monster_test_generated.h
* Provide FLATBUFFERS_BUILD_SAMPLES, FLATBUFFERS_BUILD_TESTS, FLATBUFFERS_INSTALL options (default ON) to allow conditional compilation and better integration for CMake projects including FlatBuffers in source tree