Windows fixes

* Compilation error -- round(double) isn't included on windows. 
* I've had inconsistent results compiling with mp_compile on windows, where access errors would occur, and then disappear the next time it was run. Singlethreading the build solved them. 
* Webp builds with the libwebp library name OMM, not webp.lib
* Imagemagick based tests need to pass if it's not installed, not error out. 
The actual test isn't run. Should this rather call [`skipTest()`](https://docs.python.org/2/library/unittest.html#unittest.TestCase.skipTest) than pass? Or even skip at the start of the test if not `imagemagick_available()`?