New review comment by lane-brain on void-packages repository

https://github.com/void-linux/void-packages/pull/28785#discussion_r576568406

Comment:
so as it turns out this was due to my choice to substitute the source distfiles for these libraries with the void repo-provided packages. this causes problems when I attempt to cross-compile, so I reverted to the original distfiles def that pulls the source packages for mpfr, mpc, etc.

zlib-devel however had to remain included in order to compile, mainly because of the use of --with-system-zlib flag. If it's advisable to try to see if dropping this flag works, I can go ahead and test that.