Use default cc from environment variable

[OldManYellsAtCloud]

In case the system uses a custom c compiler instead of cc (e.g. for cross-compiling), probing system features can fail or can misidentify the features due to the incorrect C compiler.

Instead of using only "cc" for probing features, check if the CC environment variable has a custom C compiler set. If it is present, use that instead of "cc". If it is not present, fall back to "cc".