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
Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:
ld: lib/auth_rpc.o:/build/source/lib/../9pfs.h:35: multiple definition of
`logfile'; 9pfs.o:/build/source/9pfs.h:35: first defined here
