Check for compiler SSE4.1 support in PCLMUL CRC code.

* cipher/crc-intel-pclmul.c: Build PCLMUL CRC implementation only if
  compiler supports PCLMUL *and* SSE4.1
* cipher/crc.c: Ditto
* configure.ac (sse41support, gcry_cv_gcc_inline_asm_sse41): New.

Fixes build with the native gcc on OpenBSD/amd64, which supports PCLMUL
but not SSE4.1.

• Signed-off-by: Jérémie Courrèges-Anglas <jca@wxcvbn.org>