In some situations, in the process of building GNU UPC, you might see the following error while going through the process of building libgcc library:

configure: error: cannot compute suffix of \
object files: cannot compile

This is indicative that just built gcc is not able to compile and create object files. The most likely reason is that GCC is not able to find gmp/mpfr/mpc dynamic link libraries. In this case you need to add locations of those libraries to your LD_LIBRARY_PATH. For example:

LD_LIBRARY_PATH="PATH_TO_GMP|MPFR|MPC_LIBS:$LD_LIBRARY_PATH" 
export LD_LIBRARY_PATH