See: #5015

The documentation in the linked PR recommends using the following when setting CC for a "make" invocation:

If the Makefile ignores environment variables, the next thing to try is command-line arguments. You can do this by overriding the build_targets attribute.
build_targets = ['CC=cc']

This is a hack: there should be a way to specify additional arguments that should be passed to make/make install.

There is a non-hack approach which is more manual: you can override the install method in the package which inherits from MakefilePackage and supply the CC setting as an argument (in which case you have to be careful to add the build/install targets yourself).