jpeg-turbo: fix race condition in make install

jpeg-turbo-1.2.0 can fail `make install` due to a race condition
on Lion using clang, where you get the error that a path exists.
Separate the make steps, and deparallelize the install.

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

Formula/jpeg-turbo.rb

@@ -13,6 +13,8 @@ class JpegTurbo < Formula
     args = ["--disable-dependency-tracking", "--prefix=#{prefix}"]
     args << "--host=x86_64-apple-darwin" if MacOS.prefer_64_bit?
     system "./configure", *args
+    system 'make'
+    ENV.j1 # Stops a race condition error: file exists
     system "make install"
   end