[libpng15] Fixed PNG_ALIGN(ED)_MEMORY and add ability to turn off ARM_NEON

runtime checks
[libpng15] Updated the INSTALL document.
[libpng15] Imported from libpng-1.5.15beta06.tar
[libpng15] Bump version to 1.5.15beta07
[libpng15] Revised INSTALL to recommend using CPPFLAGS instead of INCLUDES.

Revised scripts/makefile.freebsd to respect ZLIBDIR and ZLIBINC.