Compiler search uses a pool of workers (#10190)

- spack.compilers.find_compilers now uses a multiprocess.pool.ThreadPool to execute
  system commands for the detection of compiler versions.

- A few memoized functions have been introduced to avoid poking the filesystem multiple
  times for the same results.

- Performance is much improved, and Spack no longer fork-bombs the system when doing a `compiler find`

lib/spack/llnl/util/tty/log.py

@@ -5,6 +5,8 @@
 
 """Utility classes for logging the output of blocks of code.
 """
+from __future__ import unicode_literals
+
 import multiprocessing
 import os
 import re