GCC 10 fixes

[David Seifert]

Dear Tex Live community,
GCC 10 will be switching its default symbol emission mode from -fcommon to -fno-
common, which is the same model as used by C++ (
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85678). In effect, programmers
should not define variables in headers. Instead, they should declare them in
headers, and define them exactly once. You can emulate the GCC 10 behaviour by
compiling sources with CFLAGS="-fno-common" and noticing that texlive defines
(and not just declares) multiple variables in headers. We have a Gentoo Bug at

https://bugs.gentoo.org/706402

I have fixed all headers to just declare variables, and then define the
variables in their adjacent .c files. Find my attached patch at

https://706402.bugs.gentoo.org/attachment.cgi?id=613154

It would be nice if we could still get this included for texlive-2020, so we
don't have to carry this patch for when GCC 10 hits the ground.

Regards
David