configure.ac: add a toggle to enable the special coverity options

Coverity tries to supply system headers and fails badly at it. A bunch of _Float... sizes are pulled in by math.h but not provided anywhere. So as a workaround, let's add an option to explicitly enable coverity support that simply #defines those types to ones we do know about and let's go on hoping it'll eventually work.

See the equivalent addition to libinput in commit 8178339b5baa717.