To activate the OpenACC extensions for C/C++ and Fortran, the compile-time
flag -fopenacc must be specified. This enables the OpenACC directive
#pragma acc in C/C++ and
!$acc directives in free form,
!$acc directives in fixed form,
!$ conditional compilation sentinels in free form and
!$ sentinels in fixed form, for Fortran. The flag also
arranges for automatic linking of the OpenACC runtime library
(OpenACC Runtime Library Routines).
See https://gcc.gnu.org/wiki/OpenACC for more information.
A complete description of all OpenACC directives accepted may be found in the OpenACC Application Programming Interface manual, version 2.6.