fix pselect(2) availability detection
fix pselect(2) availability detection

Fixes https://sourceforge.net/tracker/?func=detail&aid=3106859&group_id=314802&atid=1324531

_POSIX_SELECT was probably a leftover from the autoconf days.

raboof/notion,anoduck/notion,neg-serg/notion,neg-serg/notion,dkogan/notion.xfttest,dkogan/notion,dkogan/notion,dkogan/notion,p5n/notion,raboof/notion,dkogan/notion.xfttest,p5n/notion,anoduck/notion,knixeur/notion,p5n/notion,knixeur/notion,p5n/notion,dkogan/notion,knixeur/notion,dkogan/notion,anoduck/notion,p5n/notion,knixeur/notion,anoduck/notion,anoduck/notion,dkogan/notion.xfttest,raboof/notion,dkogan/notion.xfttest,raboof/notion,neg-serg/notion,knixeur/notion,neg-serg/notion