For Darwin 6.x you need to setup some environment variables before
running "./configure":
 CFLAGS="-I/sw/include"
 LDFLAGS="-L/sw/lib"

You can combine this with the configure command like
 CFLAGS="-I/sw/include" LDFLAGS="-L/sw/lib" ./configure

Currently Darwin does not handle the "__attribute__ ((constructor))" 
entries in C files as it seems, so in your programs on Darwin you will
have to call GWEN_Init() yourself before doing anything else...