To activate the OpenMP extensions for C/C++ and Fortran, the compile-time
flag -fopenmp must be specified. This enables the OpenMP directive
#pragma omp in C/C++ and !$omp directives in free form,
c$omp, *$omp and !$omp directives in fixed form,
!$ conditional compilation sentinels in free form and c$,
*$ and !$ sentinels in fixed form, for Fortran. The flag also
arranges for automatic linking of the OpenMP runtime library
(Runtime Library Routines).
A complete description of all OpenMP directives accepted may be found in the OpenMP Application Program Interface manual, version 4.0.