[libpng16] Allow unversioned links created on install to be disabled in configure. In configure builds 'make install' changes/adds links like png.h and libpng.a to point to the newly installed, versioned, files (e.g. libpng17/png.h and libpng17.a). Three new configure options and some rearrangement of Makefile.am allow creation of these links to be disabled.