Make cmake respect CC and PATH compiler overrides

[EvanTheB]

Cmake does not respect non-system default compilers using the conventional override methods (CC env variable and PATH). https://stackoverflow.com/questions/29902862/how-to-get-cmake-to-use-the-default-compiler-on-system-path

[EvanTheB]

This was not just that I had to manually override CC after building the makefiles. CMAKE would not run because my system G++ was too old.

[kyleabeauchamp]

I've also found that this is a problem when building the bundled tar dependencies such as HTSLib, which may have their own hard-coded gcc calls in their Makefiles. See, e.g., https://github.com/bioconda/bioconda-recipes/pull/12581/