use /opt/local for gmp & mpfr

dkarm-eabi/scripts/build-gcc.sh

@@ -49,6 +49,7 @@ then
         --with-gcc --with-gnu-ld --with-gnu-as \
         --disable-shared --disable-threads --disable-win32-registry --disable-nls --disable-debug\
         --disable-libmudflap --disable-libssp --disable-libgomp \
+        --with-gmp=/opt/local --with-mpfr=/opt/local \
         --disable-libstdcxx-pch \
         --target=$target \
         --with-newlib \