m4/compiler_warnings.m4: Let -Werror be disabled

It's annoying for distributors to build with -Werror, since it means
that compiler upgrades can break the build. Let's let them disable it,
but keep it enabled by default.

m4/compiler_warnings.m4

@@ -1,11 +1,11 @@
 AC_DEFUN([NM_COMPILER_WARNINGS],
 [AC_ARG_ENABLE(more-warnings,
-	AS_HELP_STRING([--enable-more-warnings], [Maximum compiler warnings]),
-	set_more_warnings="$enableval",set_more_warnings=yes)
-AC_MSG_CHECKING(for more warnings, including -Werror)
+	AS_HELP_STRING([--enable-more-warnings], [Possible values: no/yes/error]),
+	set_more_warnings="$enableval",set_more_warnings=error)
+AC_MSG_CHECKING(for more warnings)
 if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 	AC_MSG_RESULT(yes)
-	CFLAGS="-Wall -Werror -std=gnu89 $CFLAGS"
+	CFLAGS="-Wall -std=gnu89 $CFLAGS"
 
 	for option in -Wmissing-declarations -Wmissing-prototypes \
 		      -Wdeclaration-after-statement -Wstrict-prototypes \
@@ -26,6 +26,9 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 		unset SAVE_CFLAGS
 	done
 	unset option
+	if test "x$set_more_warnings" = xerror; then
+		CFLAGS="$CFLAGS -Werror"
+	fi
 else
 	AC_MSG_RESULT(no)
 fi