Re-add C project support

[x] Fallback to rez.c when rez.cpp is absent.
[x] Mark configuration as C language when rez.c.
[x] UNIX: Default to cc compiler when C language.
[x] Propagate CFLAGS when C language.

Note that rez itself will remain written in C++. This restricts rez usage in certain fringe operating systems that lack a modern C++(17) compiler. Well, they can install a modern C++(17) compiler or use something else.

mcandre / rez

Re-add C project support #19