libdrm: Set _XOPEN_SOURCE and _GNU_SOURCE

Several POSIX extensions are used in the libdrm code (e.g., mknod and ffs).
Set _XOPEN_SOURCE and _GNU_SOURCE to something reasonable to ensure that
prototypes for these functions are available.  This is done in configure.ac
using AC_USE_SYSTEM_MACROS.  This requires autoconf 2.60 or later.  Eventually
the code should check for the existance of these defines and do something
reasonable if they are not available.

Inspired by a patch by Pauli Nieminen and suggestions from Julien Cristau.
Thanks.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>