There's no need anymore to call an own error handler directly. Instead we
register our error handler and call libgd's error functions (which will
forward). We do this regardless of compiling with the bundled or an external
libgd.
* integrate gd_arc.c into gd.c (as of gd-2.1.0-alpha1)
* rename gd_color.c to gd_color_match.c (as of gd-2.1.0-alpha1)
* remove mathmake.c (as of GD_2_0_34RC1)
* rename xbm.c to gd_xbm.c (as of gd-2.1.0-alpha1)