which is the default behavior on C99 and up.

As an extension, GNU C treats sizeof(void) as 1. MSVC doesn't.
Make it an error on GCC/Clang to avoid accidental MSVC breakage.

Otherwise using a compiler that defaults to -std=c89 or -std=gnu89 will
fail. Example:
http://www.cpantesters.org/cpan/report/abb85066-d283-11e7-9926-b2f4efb9c382

Apparently, -m32 Travis CI build was broken: -m32 was overridden by
-std=gnu99.  This fixes that.

Some people might find this handly