build system: choose gnu99 as a C standard

build system: choose gnu99 as a C standard

Apparently pre 5.1 gcc defaulted to gnu89, but we decided it was ok to
use some c99 features.

'-std=c99' by itself is not enough for notmuch to compile.

'-std=gnu99' seems to work with clang and gcc, so I'm not convinced
configuration support is needed.