Added cross compilation to CMake build system.

Brings it more into line with make based system. I've tested on 32- and 64-bit
Windows, but not yet Linux.

tests/tcctest.py

@@ -0,0 +1,15 @@
+import subprocess
+import sys
+import difflib
+
+def main():
+  reference = subprocess.check_output([sys.argv[1]])
+  compare = subprocess.check_output(sys.argv[2:])
+  failed = False
+  for line in difflib.unified_diff(reference.split('\n'), compare.split('\n'), fromfile='cc', tofile='tcc', lineterm=''):
+    failed = True
+    print line
+  sys.exit(1 if failed else 0)
+  
+if __name__ == '__main__':
+  main()