Check for err.h and work around for AIX

I was trying to compile specfem3d devel branch on AIX and I got stuck on the fact that the platform is missing err.h. So I introduced a check for and replaced the err macro from err.h by a smaller macro if err.h is not found.

Note that this has nothing to do with using the XL compiler I also have power hardware running linux and where the XL compiler is available. In this case err.h is present (part of glibc) so discriminating on the ground of the compiler used is not appropriate.