luajit: fix build

make amalg separated from make install. Without this build can fail with
more than one parallel buildjob.

Closes Homebrew/homebrew#24530.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/luajit.rb

@@ -26,7 +26,8 @@ class Luajit < Formula
     # This doesn't yet work under superenv because it removes '-g'
     args << 'CCDEBUG=-g' if build.include? 'enable-debug'
 
-    system 'make', 'amalg', 'install', *args
+    system 'make', 'amalg', *args
+    system 'make', 'install', *args
   end
 
   test do