ng.  Use environment variables in
             src/environ.h, such as HAVE_LIBPNG.
         (b) Disabling the GNU functions for redirecting formatted stream I/O
             to memory.  By default, HAVE_FMEMOPEN is enabled in src/environ.h.
         (c) Using special memory allocators (see src/environ.h).
         (d) Changing compile and runtime defaults for messages to stderr.
             The default in src/environ.h is to output info, warning and
             error messages.
         (e) Specifying the 