cmake: Allow overwriting the debug postfix of library filenames

Users may opt to change the debug suffix, e.g., from "d" to "_debug".

Rather than making CMAKE_DEBUG_POSTFIX a cache variable (which is
an antipattern in CMake), we introduce a new cache variable, named
PNG_DEBUG_POSTFIX.

Suggested-by: Diego Barrios Romero <eldruin@gmail.com>
1 file changed