detect conda gcc for -Wno-unused-but-set-variable as well

[mr-c]

Description

Allows mypyc to recognize a conda gcc compiler, so that -Wno-unused-but-set-variable may be set so that compilation does not fail.

On arm64, the conda compiler is named aarch64-conda-linux-gnu-cc and on x86_65 the conda compiler is named x86_64-conda-linux-gnu-cc

$ python -c 'from distutils import sysconfig, ccompiler; compiler = ccompiler.new_compiler(); sysconfig.customize_compiler(compiler); print(compiler.compiler[0])'

x86_64-linux-gnu-gcc

$ conda activate

(base) $ python -c 'from distutils import sysconfig, ccompiler; compiler = ccompiler.new_compiler(); sysconfig.customize_compiler(compiler); print(compiler.compiler[0])'

/home/michael/miniconda3/bin/x86_64-conda-linux-gnu-cc
# note the lack of "gcc" in the compiler name

See line 947 of https://cloud.drone.io/conda-forge/schema-salad-feedstock/6/4/2 for an example of how this issue caused a build failure

Test Plan

If desired, I can refactor the gcc testing code to its own function, and then add some unit tests.

[hauntsaninja]

Thanks!