VPC (/ninja) does not compile *.c files using `cc`, uses `cxx` instead

[Alex-gnzl]

I'm trying to integrate a third party C++ library into my project and it so happens it utilizes another small C library as part of itself.

Ninja build files produced by VPC seem to unconditionally call cxx for any source files. I solved this by breaking out the small C library into it's own project and specifying -x c to g++, however this is something that usually worked with old Makefiles because make implicitly decides to use cc for .c files.

I tried looking for examples of compiling C libs in the repository, but the only one is libspng and it's not even up to date with modern VPC.