Without the change build fails in` master` as https://hydra.nixos.org/build/247269225:
checking whether gcc supports "-Wunsigned-char"... checking how to run the C preprocessor... gcc -E
configure: error: in '/build/source':
configure: error: C preprocessor "gcc -E" fails sanity check
See 'config.log' for more details
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