Change test program to C?

[rrthomas]

I built with ENABLED_LANGUAGES=c, which seems reasonable: C++ doesn't seem to be widely used in DJGPP. The trouble is that the build script tests the compiler by building a C++ program. A simple fix would be to change to just a C program; or, build a C program, and then, if g++ was built, a C++ program.

[andrewwutw]

Fixed in f214ecc .