Allow overriding the compiler in integration tests

[davidlattimore]

While looking at #140, I noticed that libc-integration.c has tests that supposedly use clang, but as far as I can tell, only actually use clang for invoking the linker, not for actually compiling the C code. We should add a config option to specify the compiler to use to compile the code. This should probably be independent from the compiler that we use to link. See also #178.

[davidlattimore]

I forgot this was already done in b2b4551