package/m4: fix build on host with glibc-2.28

glibc-2.28 did quite some lifting in their headers, which breaks the
way some packages were detecting glibc, like gnulib.

However, packages do bundle gnulib (it was meant to be bundled),
and so does m4.

Since m4 hasn't seen the slightest commit since 2017-01-09, it is
bundling an old gnulib version, that predates glibc-2.28, and thus
breaks. It also means that upstream hasn't already fixed the issue.

Furthermore, as upstream is using a git submodule for gnulib, and
that the paths are not the same in the release tarball (in lib/)
and in the git tree (in gnulib/), we can't do a plain backport.

So, we selectively backport the two patches from gnulib upstream,
restricted to only the files that happen to be used in m4.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reported-by: c32 on IRC
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>