Improve OpenMP detection for different platforms and compilers
Improve OpenMP detection for different platforms and compilers

Currently the SciPy on Windows build gives warnings like:
```
clang-cl: warning: unknown argument ignored in clang-cl: '-print-file-name=libgomp.dylib' [-Wunknown-argument]
clang-cl: error: no input files
clang-cl: warning: unknown argument ignored in clang-cl: '-print-file-name=libgomp.so' [-Wunknown-argument]
clang-cl: error: no input files
```
for every Pythran invocation. This should improve that.

Also fix a str/bytes issue that resulted in invalid paths starting with
`b/...` because of calling str() on a bytes object.

serge-sans-paille/pythran,pombredanne/pythran,pombredanne/pythran,pombredanne/pythran,serge-sans-paille/pythran