I've exported those variables but it didn't work.

First error I get is this:
In file included from /usr/include/features.h:388:0,
                 from /usr/include/stdio.h:28,
                 from ../../../libgcc/../gcc/tsystem.h:87,
                 from ../../../libgcc/../gcc/libgcc2.c:29:
/usr/include/gnu/stubs.h:7:27: fatal error: gnu/stubs-32.h: No such file or directory

Easily fixed by doing creating a symbolic link to /usr/include/i386... in /usr/include
then I get

checking whether make sets $(MAKE)... yes
checking for i686-pc-linux-gnu-gcc... /home/maartenvd1994/gdc_building/gdc/dev/gcc-4.6.1/objdir/./gcc/xgcc -B/home/maartenvd1994/gdc_building/gdc/dev/gcc-4.6.1/objdir/./gcc/ -B/opt/gdc/i686-pc-linux-gnu/bin/ -B/opt/gdc/i686-pc-linux-gnu/lib/ -isystem /opt/gdc/i686-pc-linux-gnu/include -isystem /opt/gdc/i686-pc-linux-gnu/sys-include  
checking for C compiler default output file name...
configure: error: in `/home/maartenvd1994/gdc_building/gdc/dev/gcc-4.6.1/objdir/i686-pc-linux-gnu/libgomp':
configure: error: C compiler cannot create executables

I'm going to try a clean debian stable install in a virtual machine if I have time and try there