This required many time related include files, functions, and variables to be
tested for via CMake, and converted to macros via #cmakedefine, so that they
would be automatically set for the test-mktime.cxx file.
This includes the basic CMake infrastructure for building and executing the test
suite. Four test categories have been added - unit, system/functional, GUI, and
simgear unit tests. The test suite is run by typing 'make test_suite'.
All of the fgfs sources are included in the new run_test_suite executable,
excluding the bootstrap routine and its main() function. The test suite
currently consists of a single dummy unit test for the NasalSys subsystem, and a
single demonstration simgear/props unit test.
