Compiling with clang and gfortran

I'm trying to compile a project I recently started working on, and was asked to compile the code in clang instead of gcc. There is a CMake file for the project, and I tried to cmake the project using

cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ ../src

However an error is thrown, which I believe is because clang doesn't have a Fortran compiler and part of the project has Fortran code. Is there a way to make it use gfortran (previously used when gcc is used) when compiling the Fortran code, and clang/clang++ for the rest?

The part of the cmake file which I think is relevant is:

enable_language(Fortran)

string(REGEX MATCH gfortran HAVE_GFORTRAN ${CMAKE_Fortran_COMPILER})
string(REGEX MATCH xlf HAVE_XLF ${CMAKE_Fortran_COMPILER})
string(REGEX MATCH pg77 HAVE_PG77 ${CMAKE_Fortran_COMPILER})
string(REGEX MATCH g77 HAVE_G77 ${CMAKE_Fortran_COMPILER})
string(REGEX MATCH ifort HAVE_ifORT ${CMAKE_Fortran_COMPILER})
string(REGEX MATCH f77 HAVE_F77 ${CMAKE_Fortran_COMPILER})

if(HAVE_GFORTRAN)
  set(F_LIBRARY gfortran CACHE string "fortran library")
  find_library(F_LIBRARY NAMES gfortran)
  set(FORTRAN_UNDERSCORE end CACHE string "What type of fortran underscore style - linux,end,none")
  set(DEF_FORTRAN_UNDERSCORE "#define FORTRAN_UNDERSCORE_END")
elseif(HAVE_XLF)
  set(F_LIBRARY xlf90 CACHE string "fortran library")
  find_library(F_LIBRARY NAMES xlf90)
  set(FORTRAN_UNDERSCORE none CACHE string "What type of fortran underscore style - linux,end,none")
  set(DEF_FORTRAN_UNDERSCORE "#define FORTRAN_UNDERSCORE_NONE")
elseif(HAVE_PGF77)
  set(F_LIBRARY pgftnrtl CACHE string "fortran library")
  find_library(F_LIBRARY NAMES pgftnrtl)
  set(FORTRAN_UNDERSCORE end CACHE string "What type of fortran underscore style - linux,end,none")
  set(DEF_FORTRAN_UNDERSCORE "#define FORTRAN_UNDERSCORE_END")
elseif(HAVE_G77)
  set(F_LIBRARY g2c CACHE string "fortran library")
  find_library(F_LIBRARY NAMES g2c)
  set(FORTRAN_UNDERSCORE linux CACHE string "What type of fortran underscore style - linux,end,none")
  set(DEF_FORTRAN_UNDERSCORE "#define FORTRAN_UNDERSCORE_LINUX")
elseif(HAVE_ifort)
  set(F_LIBRARY ifcore CACHE string "fortran library")
  find_library(F_LIBRARY NAMES ifcore)
  set(FORTRAN_UNDERSCORE end CACHE string "What type of fortran underscore style - linux,end,none")
  set(DEF_FORTRAN_UNDERSCORE "#define FORTRAN_UNDERSCORE_END")
elseif(HAVE_F77)
  set(FORTRAN_UNDERSCORE end CACHE string "What type of fortran underscore style - linux,end,none")
  set(FORTRAN_LIBRARY "" CACHE string "fortran library")
  set(DEF_FORTRAN_UNDERSCORE "#define FORTRAN_UNDERSCORE_END")
endif()

# f77 on redstorm currently an exception - doesn't need it 
if(NOT F_LIBRARY AND NOT HAVE_F77)
  message(FATAL_ERROR "Cannot find fortran library")
endif(NOT F_LIBRARY AND NOT HAVE_F77)

Terminal output says that this is threw an error when I tried cmake:

message(FATAL_ERROR "Cannot find fortran library")

Please let me know if I need to post any more info. Many thanks in advance!

In order not to leave the question open:

The variable CMAKE_Fortran_COMPILER must be set to the name of the compiler executable (gfortran) if cmake is not able to determine it automatically.