Fix build failure on Mac OSX 10.5, posix_memalign was introduced in 10.6

We fixed a related issue in 51e8674:
  Call _mm_malloc if mm_malloc.h header is found, otherwise call
  posix_memalign if this function was found, otherwise call malloc.

This patch had been dropped after 0.12 because of an incompatible change
with OCCT 6.6.0.
This commit modifies src/Image/Image_PixMap.cxx to match the version
in OCE 0.12.

Fixes issue #378.