cmake: Find our PNG dependency on behalf of consumers

cmake/adios2-config-common.cmake.in

@@ -47,6 +47,11 @@ if(NOT @BUILD_SHARED_LIBS@)
     find_dependency(MGARD)
   endif()
 
+  set(ADIOS2_HAVE_PNG @ADIOS2_HAVE_PNG@)
+  if(ADIOS2_HAVE_PNG)
+    find_dependency(PNG)
+  endif()
+
   set(ADIOS2_HAVE_DataSpaces @ADIOS2_HAVE_DataSpaces@)
   if(ADIOS2_HAVE_DataSpaces)
     find_dependency(DataSpaces)