[Boost-build] Better caching of configure checks

Subject: [Boost-build] Better caching of configure checks
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2012-10-13 21:12:26

AMDG

The attached patch implements a cache that
stores the results of all configure checks.
This reduces the no-op build time
for msvc-11.0 in BOOST_ROOT with no extra
options from 8 seconds to 3 seconds for me.

The cache is stored in a file called
project-cache.jam which is created in
the directory specified by --build-dir
(if this flag is passed) or in the
directory containing Jamroot.

The --reconfigure option causes the
cache to be regenerated from scratch.
I think that passing -a should also clear
the cache, but I put that off, since
the engine doesn't expose most of its
flags to Jam code.  (I'd rather not
duplicate the option parsing logic
either.)

I've tested this with msvc-9.0express,
msvc-10.0express, and msvc-11.0 on
Windows 7 and with gcc-4.5.1, gcc-4.7.2,
and clang on linux.

Any thoughts on this before I commit it?

In Christ,
Steven Watanabe