[CMake] Cross-compiling, CMAKE_C_FLAGS, and configure-time compiler tests

[Johan Björk]

Hi Justin,

I'm very unsure if this is the correct solution, but it worked for me. I
haven't been able to find any good documentation stating how the CMakeCache
interacts with other parts of CMake.

My assumption is that since CMAKE_C{XX}_FLAGS is supposed to allow the user
to set optional compilation flags, it prefers the value in the cache versus
the one from the toolchain file.