Compilation warning with gcc 4.8.5

Compiling the latest swift master branch (i.e., d1b43ed) gives warnings, which by default are treated as error and stop the compilation.

The gcc version:

$> gcc --version
gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-39)
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The configure command:

../configure --enable-ipo --enable-debug --with-hdf5 --with-fftw --with-parmetis=/home/rtobar/local/parmetis/hyrmine/4.0.3/ --with-gsl=/opt/bldr/local/numerics/gsl/2.5 --with-tbbmalloc --with-hydro=sphenix --with-kernel=wendland-C2 --with-subgrid=EAGLE-XL --with-velociraptor=/home/rtobar/scm/git/swiftsim/build/../../VELOCIraptor-STF/builds/native/src MPI_CC=/opt/bldr/local/distributed/openmpi4/4.0.1/bin/mpicc

And the offending compilations:

[...]
libtool: compile:  mpicc -std=gnu99 -DHAVE_CONFIG_H -I. -I../../src -I.. -I/opt/bldr/local/storage/hdf5/1.10.5/include -I/opt/bldr/local/compression/zlib/1.2.11/include -I/opt/bldr/local/compression/szip/2.1.1/include -I/opt/bldr/local/compression/szip/default/include -I/opt/bldr/local/compression/zlib/default/include -I/usr/include -I/opt/bldr/local/numerics/gsl/2.5/include -fopenmp -DWITH_MPI -I/home/rtobar/local/parmetis/hyrmine/4.0.3//include -g -O0 -flto -gdwarf-2 -fvar-tracking-assignments -O3 -fomit-frame-pointer -malign-double -fstrict-aliasing -ffast-math -funroll-loops -march=core-avx2 -mavx2 -pthread -fopenmp -Wall -Wextra -Wno-unused-parameter -Wshadow -Wstrict-prototypes -MT cooling/COLIBRE/mpi-cooling_tables.lo -MD -MP -MF cooling/COLIBRE/.deps/mpi-cooling_tables.Tpo -c ../../src/cooling/COLIBRE/cooling_tables.c -o cooling/COLIBRE/mpi-cooling_tables.o >/dev/null 2>&1
In file included from ../../src/runner.h:31:0,
                 from ../../src/engine.h:45,
                 from ../../src/active.h:27,
                 from ../../src/runner_doiact_grav.c:26:
../../src/gravity_cache.h: In function ‘gravity_cache_populate’:
../../src/gravity_cache.h:229:0: warning: ignoring #pragma omp simd [-Wunknown-pragmas]
 #pragma omp simd
 ^
../../src/gravity_cache.h: In function ‘gravity_cache_write_back’:
../../src/gravity_cache.h:489:0: warning: ignoring #pragma omp simd [-Wunknown-pragmas]
 #pragma omp simd
 ^
[...]
mv -f cooling/COLIBRE/.deps/mpi-cooling_tables.Tpo cooling/COLIBRE/.deps/mpi-cooling_tables.Plo
../../src/runner_doiact_grav.c: In function ‘runner_dopair_grav_pm_full’:
../../src/runner_doiact_grav.c:886:0: warning: ignoring #pragma omp simd [-Wunknown-pragmas]
 #pragma omp simd
 ^
../../src/runner_doiact_grav.c: In function ‘runner_dopair_grav_pm_truncated’:
../../src/runner_doiact_grav.c:1029:0: warning: ignoring #pragma omp simd [-Wunknown-pragmas]
 #pragma omp simd
 ^
[...]

The first two warnings repeat over and over as they appear on a header file.

As in #698 (closed) I can workaround this by using --enable-compiler-warnings=yes.

I have a local fix based on checking the value of the _OPENMP macro. If you think this is a desirable solution (a different one would be to fully enable/disable OpenMP after checking the OpenMP version in configure.ac) I can submit a pull request with the changes.