Add libpng as a dependency for freetype for static builds

Newer versions of Freetype requires libpng as well.
diff --git a/cmake/StaticBuild.cmake b/cmake/StaticBuild.cmake
index 6db0e14..06883c6 100644
--- a/cmake/StaticBuild.cmake
+++ b/cmake/StaticBuild.cmake
@@ -91,7 +91,7 @@
       if(${CMAKE_SYSTEM_NAME} MATCHES "SunOS")
         set(FLTK_LIBRARIES "${FLTK_LIBRARIES} ${X11_Xcursor_LIB} ${X11_Xfixes_LIB} -Wl,-Bstatic -lXft -Wl,-Bdynamic -lfontconfig -lXrender -lXext -R/usr/sfw/lib -L=/usr/sfw/lib -lfreetype -lsocket -lnsl")
       else()
-        set(FLTK_LIBRARIES "${FLTK_LIBRARIES} -Wl,-Bstatic -lXcursor -lXfixes -lXft -lfontconfig -lexpat -lfreetype -lbz2 -lXrender -lXext -lXinerama -Wl,-Bdynamic")
+        set(FLTK_LIBRARIES "${FLTK_LIBRARIES} -Wl,-Bstatic -lXcursor -lXfixes -lXft -lfontconfig -lexpat -lfreetype -lpng -lbz2 -lXrender -lXext -lXinerama -Wl,-Bdynamic")
       endif()
 
       set(FLTK_LIBRARIES "${FLTK_LIBRARIES} -lX11")