Building on Mac OS complains about prebound libraries when linking.

[sdlime]

It's benign but annoying. The exact error is:

ld: warning prebinding disabled because dependent library:
/usr/local/lib/libgd.2.dylib is not prebound

I don't know if it's a problem with GD or not, but GCC suggests linking with the
option -bind_at_load.

Adding to the EXE_LDFLAGS variable setting in the Makefile fixes things. Would
be nice if conifigure could handle this. I remember seeing something in the GDAL
build scripts to deal with this. However, the change is best made by someone
more familiar with autoconf or automake than I.

Steve