LU-5018 libcfs: don't depend on configure for lustre_user.h

Don't depend on configure to detect the presence of <linux/types.h>
in order to define the __u8, __u16, __u32, and __u64 types.  Instead,
just define these types directly if <linux/types.h> is not included
already.

The <linux/types.h> header is available on Linux systems with GCC
installed, which is required to build Lustre anyway.  On non-Linux
systems they should create an appropriate <linux/types.h> compat
header in the include path.

Lustre-change: http://review.whamcloud.com/10258
Lustre-commit: 241cdb8ba008d47fd96f0b6529797ce922e2b635

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Ida0915fe1b5829f3378e459c782844cf9df060e6
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/11840
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>