CMake/build.cmake : Fix for CMake 3.0.2.

Previous versions of CMake allowed targets names like "tests/win32_test"
but CMake 3.0.2 doesn't. The corresponding target name for the new CMake
is "win32_test", but putting the resulting executable in the tests/
directory requires something like:

    add_executable (win32_test tests/win32_test.c)
    set_target_properties (win32_test
                           PROPERTIES RUNTIME_OUTPUT_DIRECTORY tests)