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
Clean spatial picture alloc code
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++
remove duplicate function
Remove stub interfaces for hw decoding

There is no implementation available for actually doing decoding
in HW.
Merge pull request #283 from mstorsjo/extend-c-test

Test more aspects of the C interface, fix using the C interface on MSVC
Merge pull request #285 from volvet/develop_b

remove duplicated function
Merge pull request #284 from volvet/clean_spatial_picture_code

Clean spatial picture alloc code
fix level_prefix too large in residual decoding
use forward declaration to replace type cast
Merge pull request #288 from volvet/develop_b

use forward declaration to replace type cast
move alloc spatial picture and free spatial picture to CWelsPreProcess
add update spatial pictures method too CWeslPreProcess
move spatial picture related variable to CWelsPreProcess
correct code style