Set language linkage to "C" when compiling C++

[tclamb]

Using mwccgap as-is within a C++ project fails with the following signature:

Exception processing lighting.cpp: func_08860760 not found in src/lighting.cpp
Traceback (most recent call last):
  File "./tools/mwccgap/mwccgap.py", line 64, in main
    process_c_file(
  File "./tools/mwccgap/mwccgap/mwccgap.py", line 128, in process_c_file
    raise Exception(f"{function} not found in {c_file}")
Exception: func_08860760 not found in src/lighting.cpp

Wrapping every INCLUDE_*() statement with extern "C" works around this issue, but adds friction to the workflow.

This change adds the typical #ifdef __cplusplus-guarded language linkage statements to the preprocessed c output.

[mkst]

Sweet, happy for me to merge this?

[tclamb]

Yeah, we're good to go.