autoconf detection of label support

autoconf detection of label support

acinclude.m4 now has struct label support detection and sets
HAVE_STRUCT_LABEL_SUPPORT in afsconfig.h.  Obviates complicated
ifdefs in various src/afs/ and src/rx/ files.  Must run regen.sh
to generate new configure and afsconfig.h.in, then configure will
detect label support and set afsconfig.h appropriately.  Not sure
what to do for Windows, but should be a step in the right
direction.

Fixes issue with compilation of libuafs on IRIX and sunpro by accurately
detecting label support.

Change-Id: I092eda47c4cfcc2517dc39f5e48b5038c413cb01
Change-Id: I2ff8449b28db645d9cfd27796b5f89e5c328a2b2
Reviewed-on: http://gerrit.openafs.org/2516
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>