52.2.1-alt2 can be built in parallel, I've noticed no problems, but a speedup:

From 0658fcd46c08c35ed4467f1f78211ec5033db973 Mon Sep 17 00:00:00 2001
From: Ivan Zakharyaschev <imz@altlinux.org>
Date: Thu, 5 Apr 2018 18:16:13 +0300
Subject: [PATCH] %%make_build for parallelization

---
 mozjs52.spec | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mozjs52.spec b/mozjs52.spec
index e5a8790..69d6208 100644
--- a/mozjs52.spec
+++ b/mozjs52.spec
@@ -101,7 +101,7 @@ export PYTHON=/usr/bin/python
 	--with-system-zlib \
 	%{?_with_system_icu:--with-system-icu} \
 	--with-intl-api
-%make
+%make_build
 
 %install
 cd js/src
-- 
2.10.4