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/AddUserEnv.py

@@ -0,0 +1,38 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import os
+import shlex
+
+from beast.env.ReadEnvFile import read_env_file
+from beast.util.String import is_string
+from beast.util.Terminal import warn
+
+_BAD_VARS_ERROR = """
+the following variables appearing in %s were not understood:
+  %s"""
+
+def add_user_env(env, dotfile, print=print):
+    df = os.path.expanduser(dotfile)
+    try:
+        with open(df, 'r') as f:
+            dotvars = read_env_file(f.read())
+    except IOError:
+        if os.path.exists(df):
+            warn("Dotfile %s exists but can't be read." % dotfile, print)
+        dotvars = {}
+
+    bad_names = []
+    for name, value in dotvars.items():
+        if name in env:
+            if is_string(env[name]):
+                env[name] = value
+            else:
+                env[name] = shlex.split(value)
+        else:
+            bad_names.append(name)
+    if bad_names:
+        error = _BAD_VARS_ERROR % (dotfile, '\n  '.join(bad_names))
+        warn(error, print)
+
+    for name, default in env.items():
+        env[name] = os.environ.get(name, default)