- remove --unix-qstart-libpng:
  the internal libpng is static lib so this option is pointless
- clean up duplicative libpng checks
- replace usage of ENABLE_QUICKSTART_LIBPNG with ENABLE_HEADLESS,
  which is the one case where it could still be disabled

Change-Id: Ie2e2c7e4a2396572742fdf056bf23c4271f04811