Use libdevq where libudev is required on Linux
configure.ac: Don't check for stddef.h presence in Clang include dir

On FreeBSD, the lang/clang* ports don't install this header and
/usr/include/stddef.h is ok.
Work around for clang 3.4 which fails to build Clover

See:
  https://bugs.freedesktop.org/show_bug.cgi?id=74098#c3