mktemp’s template argument is required on OS X

And possibly others. Fixes #337.
Use gcc -std=c99 instead of c99 on OS X

Automatic checking in Makefile. Fixes #308 and #340.
Remove install note about OS X

Should work “out of the box” now. See #340.
Merge remote-tracking branch 'promy/master'
Merge remote-tracking branch 'origin/master' into dl