libnuma, immintrin.h detection in configure script

Add a check for the numa.h header (not just the libnuma binary) and a more robust check for vector.h's inclusion of immintrin.h (currently false negative). Also enable example/main.c's -o option - handled, but absent from getopt call.

This is the last of the non-vectorisation changes from local_position_cache.

mentioned in commit a7baf53d

Status changed to merged

Looks good, so accepted.