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/ReadEnvFile.py

@@ -0,0 +1,34 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import json
+import re
+
+from beast.util import String
+from beast.util.Terminal import warn
+
+ENV_LINE_MATCH = re.compile(r'(?: export \s+)? \s* ([^=\s]*) \s* = (.*)',
+                            re.VERBOSE)
+
+def read_env_file(data, print=print):
+    try:
+        return json.loads(data)
+    except ValueError:
+        pass
+
+    bad_lines = []
+    results = {}
+    for number, raw_line in enumerate(data.splitlines()):
+        line = String.remove_comment(raw_line).strip()
+        if line:
+            match = ENV_LINE_MATCH.match(line)
+            if match:
+                name, value = match.groups()
+                results[name.strip()] = String.remove_quotes(value.strip())
+            else:
+                bad_lines.append([number, raw_line])
+    if bad_lines:
+         warn("Didn't understand the following environment file lines:", print)
+         for number, line in bad_lines:
+             print('%d. >>> %s' % (number + 1, line))
+
+    return results