[build] Check if GLOB_PERIOD exists

[Melkor333]

As discussed briefly in Zulip. Not sure what a "peoper" solution is, defining GLOB_PERIOD as 0 is probably wrong :)

[andychu]

OK thanks -- actually I think it should never be 0 in real life, so 0 is a good test of whether it exists! Then we can print a warning on shopt -s dotglob

I also did the same for FNM_EXTMATCH, the extended glob issue