Replace cppunit with the googletest framework

The GTest framework has a more active support base, and natively
supports CMake. Introduce it as an external dependency (using CMake's
ExternalProject_Add), replacing cppunit and porting the associated unit
tests.

code/AssxmlExporter.cpp

@@ -51,6 +51,8 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #	include "../contrib/zlib/zlib.h"
 #endif
 
+#include <time.h>
+
 #ifndef ASSIMP_BUILD_NO_EXPORT
 #ifndef ASSIMP_BUILD_NO_ASSXML_EXPORTER