roken: Check for bswap16 and bswap32 defines

roken: Check for bswap16 and bswap32 defines

Our imported roken.h provides definitions for bswap16 and bswap32,
but configure doesn't check to see if those functions already exist.
On operating systems where they do, and where the header containing
them is included in our build, this results in compilation errors.

Add a configure test for bswap16 and bswap32, which is complicated
by the fact that they may be macros, or static inline functions, so
a simple CHECK_FUNCS won't work.

Change-Id: Iae3411bf4bb22af8cfd770e01026a28e1863e078
Reviewed-on: http://gerrit.openafs.org/3613
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>