Improvements to scons build for beast.

* Common code extracted to Python directories.
* Read ~/.scons file for scons environment defaults.
* Override scons settings with shell environment variables.
* New "tags" for debug, nodebug, optimize, nooptimize builds.
* Universal platform detection.
* Default value of environment variables set through prefix dictionaries.
* Check for correct Boost value and fail otherwise.
* Extract git describe --tags into a preprocesor variable, -DTIP_BRANCH
* More colors - blue for unchanged defaults, green for changed defaults, red for error.
* Contain unit tests for non-obvious stuff.
* Check to see that boost libraries have been built.
* Right now, we accept both .dylib and .a versions but it'd be easy to enforce .a only.

python/beast/env/AddCommonFlags.py

@@ -0,0 +1,11 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+from beast.util import Boost
+from beast.util import Git
+
+def add_common_flags(env):
+    git_flag = '-DTIP_BRANCH="%s"' % Git.describe()
+    env['CPPFLAGS'] = '%s %s' % (env['CPPFLAGS'], git_flag)
+    env['CPPPATH'].insert(0, Boost.CPPPATH)
+    env['LIBPATH'].append(Boost.LIBPATH)
+    env['BOOST_HOME'] = Boost.CPPPATH