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

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.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>