Do not install low-level C parallel grades by default.

configure.ac:
compiler/handle_options.m:
     As above.

compiler/handle_options.m

@@ -852,6 +852,9 @@ check_for_incompatibilities(Globals, OpMode, !Specs) :-
         HighLevelCode = no,
         Parallel = yes
     then
+        % When the bug referred to below is fixed, don't forget to
+        % to re-enable parallel low-level C grades in the configure
+        % script.
         LowParSpec =
             [words("Sorry, but the implementation of parallelism"),
             words("for low-level C code currently has a bug that"),

configure.ac

@@ -3806,7 +3806,11 @@ fi
 if test "$enable_par_grades" = yes -a "$CFLAGS_FOR_THREADS" != "" \
     -a "$MR_HAVE_PTHREAD_H" = 1
 then
-    LIBGRADES="$LIBGRADES hlc.par.gc $BEST_LLDS_BASE_GRADE.par.gc.stseg"
+    # We should also add $BEST_LLDS_BASE_GRADE.par.gc.stseg to LIBGRADES
+    # here, but that is currently disabled until a bug in parallel low
+    # level-C grades is fixed. (See check_for_incompatibilities/4 in
+    # in compiler/handel_options.m.)
+    LIBGRADES="$LIBGRADES hlc.par.gc"
 fi
 
 # Add the .stseg (stack segments) version of the library.