Build fails with latest ctypesgen

Since ctypesgen 0.1.1 (now 1.0.0) it has installed as "ctypesgen" instead of "ctypesgen.py" so CMakeLists.txt can't find the executable.

The fix is trivial:

```
FIND_PROGRAM(CTYPESGEN_FOUND ctypesgen.py)
IF (NOT CTYPESGEN_FOUND)
  FIND_PROGRAM(CTYPESGEN_FOUND ctypesgen)
ENDIF()
```
If that's acceptable I'll push a PR.

Thanks