passing arguments to build-gcc

[i-n-g-o]

hi.

with this change it was able to pass additional arguments to gccs configure script. on osx i needed to specify the path for gmp, mpfr and mpc...

i could compile everything in on go with: ./build-elf.sh lx106 --with-mpc=/opt/local --with-mpfr=/opt/local --with-gmp=/opt/local