Much of this may be unnecessary if the gfortran dmg is used.
Install GMP
Install MPFR
http://www.mpfr.org/mpfr-current/#download
Install MPC
http://www.multiprecision.org/index.php?prog=mpc
Installing gfortran
https://gcc.gnu.org/wiki/GFortranBinaries
Puts gfortran in /usr/local/bin/gfortran.
Install gsl
Install gsl globally.
Building the source
cd ~/code/multinest
tar -xvf multinest.tar
Comment out the mpi lines.
Then make.