1. support .c in makefile generator. 2. add test to check C interfaces.
Merge pull request #281 from jwwang/add_gtests

1. support .c in makefile generator. 2. add test to check C interfaces.
Regenerate target makefiles after the latest mktargets.py changes
Set CC wherever we set CXX
Merge pull request #282 from mstorsjo/update-make-for-c-test

Update makefiles after adding a C interface test
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