The simd OpenMP pragma was introduced in OpenMP 4.0, but the code
neither checked for the OpenMP version at configure time, nor it checks
for it at build time. This causes compilers with older OpenMP versions
to produce warnings when building the code, which in turn causes
compilation errors when configuring the project by default.

This commit adds a check on the _OPENMP macro value in all places where
the simd OpenMP pragma is used. Alternatively one could do this check at
configure time and disallow compilation altogether if the requirement is
not met.

Signed-off-by: Rodrigo Tobar <rtobar@icrar.org>