Environment allowing Fortran to call a C function

I'm trying to install a large atmospheric model (WRF) on my newly set up Centos 7 machine. To start the installation process, the model's developers give a series of environmental tests. My set up fails the fourth one--Fortran Calling a C Function. (I passed the first three: fixed and free format Fortran and C only.) This test consists of two small programs. First, TEST_4_fortran+c_c.c:

::::::::::::::
TEST_4_fortran+c_c.c
::::::::::::::
#ifndef CRAY
# ifdef NOUNDERSCORE
#      define C_TEST c_test
# else
#   ifdef F2CSTYLE
#      define C_TEST c_test__
#   else
#      define C_TEST c_test_
#   endif
# endif
#endif
#include 

int C_TEST ( float *xx, int *ii )

{
 printf("   C function called by Fortran\n" ) ;
 printf("   Values are xx = %5.2f and ii = %d \n", *xx, *ii ) ;
 return(0) ;
}

Compiling it "gcc -c -m64 TEST_4_fortran+c_c.c" went fine with no errors. The second program, TEST_4_fortran+c_f.f90:

PROGRAM foo
   INTEGER :: ii
   REAL    :: xx

   ii = 1
   xx = 2

   CALL c_test ( xx , ii )

   print *,'SUCCESS test 4 fortran calling c'

END PROGRAM foo

Compiling gfortran -c -m64 TEST_4_fortran+c_f.f90 also had no errors. When I attempt to execute the fortran gfortran -m64 TEST_4_fortran+c_f.o I get:

TEST_4_fortran+c_f.o: In function `MAIN__':
TEST_4_fortran+c_f.f90:(.text+0x2f): undefined reference to `c_test_'
collect2: error: ld returned 1 exit status

I just installed Centos 7, so everything is "out of the box."