mapserver 5.0.0-beta2 and AGG detection in configure.in

[Even Rouault]

Currently in configure.in, if you specifiy "--with-agg" or "--with-agg=yes" 
there is a check for gpc_polygon_clip to enable AGG build. 
AC_CHECK_LIB(agg, gpc_polygon_clip, AGG_ENABLED=-DUSE_AGG,,,)

However, GPC has a non free licence ("free for non-commercial use"). See 
http://www.antigrain.com/license/index.html. So, my build of AGG doesn't 
include GPC.

I've removed the check for gpc_polygon_clip in configure.in and mapserver 
5.0.0-beta2 builds fine.