Update fontconfig and libpng versions
Similar to !557 (merged), this MR cleans out some legacy fontconfig and libpng cruft.
There was a check for libpng 1.2.9. Updated to require 1.4.0 (released 2010).
Looking at the fontconfig checks, then newest feature is FC_FONT_VARIATIONS
added to 2.12.91. Updated the minimum to 2.13.0 (released 2018).