with structuredAttrs lists will be bash arrays which cannot be exported
which will be a issue with some patches and some wrappers like cc-wrapper
this makes it clearer that NIX_CFLAGS_COMPILE must be a string as lists
in env cause a eval failure
Without the change the build against upstream gcc-10 build fails as:
ld: smScSema.o:/build/source/compiler/smScSema.c:135:
multiple definition of `charset'; options.o:/build/source/compiler/options.c:16: first defined here
Use -fcommon as a workeround until upstream updates the code.
