Test for headers and library for FONTFORGE_ARG_WITH_LIBPNG()

Replaced pkg-config check with a check for headers and library.
./configure does a hard-stop if user specifies '--with-png' and there is
no library or developer files available. Fixed *.pc dependency list.
Missed adding to test != xno
Merge pull request #1432 from JoesCat/check_headers_and_libraries_2

Test for headers and library for FONTFORGE_ARG_WITH_LIBPNG()