Makefile.global: drop -std=gnu99. C11 (or later) compiler required

Makefile.global: drop -std=gnu99. C11 (or later) compiler required

Since October 2018 building notmuch has actually required compiler
that knows C11.

Also this -std=gnu99 was not used in code compiled by configure,
so in theory this could have caused problems...

...but no related reports have been sent, perhaps ever.

Both gcc and clang has been shipping compilers supporting C11
(or later) by default for more than four years now.

Therefore, just dropping -std=gnu99 (and not checking C11
compatibility for now, for simplicity) is easiest to do,
and removes inconsistency between configure and build time
compilations.