Don't try to include stdbool.h on MSVC

MSVC only got stdbool.h in MSVC 2013; provide a manual typedef
for this compiler instead.

The C interface test makes sure that this doesn't break the ABI.
Add a test that verifies that the bool typedef in C is ABI compatible with C++
Merge pull request #283 from mstorsjo/extend-c-test

Test more aspects of the C interface, fix using the C interface on MSVC