configure: don't check for arpa/nameser_compat.h twice

Nuke redundant check and move the real one into the common
AC_CHECK_HEADERS stanza.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
configure: fix F_SETPIPE_SZ detection

Currently CEPH_HAVE_SETPIPE_SZ is not set even if F_SETPIPE_SZ is
available, because AC_COMPILE_IFELSE test program as written always
fails to compile.  F_SETPIPE_SZ is a macro, so use AC_EGREP_CPP which
works on the preprocessor output instead of trying to compile.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
common/buffer: fix build breakage for CEPH_HAVE_SETPIPE_SZ

common/buffer.cc fails to build if CEPH_HAVE_SETPIPE_SZ is defined.
Fix it.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1204 from ceph/wip-fsetpipesz-fix

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>