SIMH 3.9 won't compile w/o deparallelize; also added --HEAD

Closes Homebrew/homebrew#14583.

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

Formula/simh.rb

@@ -5,17 +5,19 @@ class Simh < Formula
   url 'http://simh.trailing-edge.com/sources/simhv39-0.zip'
   sha1 '1de3938f0dcb51d55b0e53aea8ae9769ccc57bdb'
   version '3.9-0'
+  head 'https://github.com/simh/simh.git'
 
   # After 3.9-0 the project moves to https://github.com/simh/simh
   # It doesn't actually fail, but the makefile queries llvm-gcc -v --help a lot
   # to determine what flags to throw.  It is simply not designed for clang.
   # Remove at the next revision that will support clang (see github site).
   fails_with :clang do
-    build 318
+    build 421
     cause 'The program is closely tied to gcc & llvm-gcc in this revision.'
   end
 
   def install
+    ENV.deparallelize unless build.head?
     inreplace 'makefile', 'GCC = gcc', "GCC = #{ENV.cc}"
     inreplace 'makefile', 'CFLAGS_O = -O2', "CFLAGS_O = #{ENV.cflags}"
     system "make USE_NETWORK=1 all"