[m-rev.] diff: replace erroneous pragma with #error directive

[Julien Fischer]

Replace erroneous pragma with #error directive.

runtime/mercury_atomic_ops.h:
     As above; on aarch64-apple-darwin we were *not* getting the error about
     memory fence operations not being defined when using clang as the C
     compiler; compilation was instead aborting later due to MR_SFENCE
     not being defined.

Julien.

diff --git a/runtime/mercury_atomic_ops.h b/runtime/mercury_atomic_ops.h
index ef6719c..f9806eb 100644
--- a/runtime/mercury_atomic_ops.h
+++ b/runtime/mercury_atomic_ops.h
@@ -580,7 +580,7 @@ MR_EXTERN_INLINE MR_bool    MR_atomic_dec_and_is_zero_uint(

  #else

-    #pragma error "Please implement memory fence operations "               \
+    #error "Please implement memory fence operations " \
          "for this compiler/architecture"

  #endif