[libpng15] 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.