Below are the instruction to compile VASP version 5.3.5 with gfortran. First step is to build the vasp.5.lib. Copy the linux_gfortran makefile:

cp makefile.linux_gfortran makefile

We are going to use gfortran, so make sure it is installed. There are several package management systems for OS X; I personally like Homebrew. One point that took me a while to realize was that OS X by default has a case insensitive file system, so preclib.F and preclib.f are the same file. VASP preprocesses the .F files and creates .f files, basically overwriting the original ones. We will need to fix that. Make the following changes to the makefile:

CPP = gcc -E -P -x c $*.F >$*.f90
FFLAGS = -O1 -cpp

and compile by typing: make and ignore any warnings. It should build the library libdmy.a. Now change over to the main folder vasp.5.3 and copy over makefile.linux_gfortran and make the following changes to the file.

FC=gfortran
CPP_ = ./preprocess <$*.F | /usr/bin/cpp -P -C -traditional -w >$*$(SUFFIX)
FFLAGS = -ffree-form -ffree-line-length-0
BLAS= -framework vecLib

Save the file and type make. Unfortunately, the code is not fully compatible with gfortran, so we will need to make several fixes. One of them is a space between a function call and (. Run the following command to fix files with errors.

for file in dfast.F choleski2.F dos.F broyden.F fock.F force.F rot.F main.F; do
  sed -i.bak s/CALLMPI\ \(/CALLMPI\(/g $file; rm $file.bak 
done
sed -i.bak s/CALLMPI_C\ \(/CALLMPI_C\(/g main.F; rm main.F.bak

Replace the USE us lines in us.F so that it looks like

1462 USE constant
1463 USE us, only : setdij_
1464 IMPLICIT NONE
2695 USE wave
2696 USE us, only : augmentation_charge
2697
2698 IMPLICIT NONE

Now, running make should hopefully compile it.