volk: add check for posix_memalign ...

... for OSs that do not directly set _XOPEN_SOURCE or _POSIX_C_SOURCE; define and use the new variable when found. This fix is OS generic: if posix_memalign is found, use it.

This pull is in response to the GR thread about volk and posix_memalign not playing nicely on OSX. This change fixes that issue for OSX 10.8+ (maybe 10.7, too; I haven't checked). That said, GR issue 710 is still relevant for OSX 10.6- and maybe 10.7.  I'm glad Apple got their stuff together for this function; I wish they would do so others, too, but I guess 1 step at a time ... - MLD