problems with make.conf

Hi!

I've a problem compiling the port graphics/libGLw. The port's Makefile 
contains the line

CFLAGS+=-D__GLX_MOTIF=1

As long as I don't set my own CFLAGS in /etc/make.conf, the port compiles 
fine. But if I set my own CFLAGS in /etc/make.conf, the CFLAGS+= line in the 
Makefile seems to be ignored (or overwritten by my settings).

How do I correctly use the /etc/make.conf file? How can I use my own settings?

Thanks,
Christian.