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 build fails on upstream gcc-10 as:
ld: plugins/vnc/vinagre_vinagre-vinagre-vnc-tab.o:plugins/vnc/vinagre-vnc-connection.h:29:
multiple definition of `scaling_command_line';
vinagre/vinagre_vinagre-vinagre-reverse-vnc-listener.o:plugins/vnc/vinagre-vnc-connection.h:29: first defined here
