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_test.py

@@ -0,0 +1,51 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+from unittest import TestCase
+from beast.env.ReadEnvFile import read_env_file
+
+from beast.util import Terminal
+Terminal.CAN_CHANGE_COLOR = False
+
+JSON = """
+{
+  "FOO": "foo",
+  "BAR": "bar bar bar",
+  "CPPFLAGS": "-std=c++11 -frtti -fno-strict-aliasing -DWOMBAT"
+}"""
+
+ENV = """
+# An env file.
+
+FOO=foo
+export BAR="bar bar bar"
+CPPFLAGS=-std=c++11 -frtti -fno-strict-aliasing -DWOMBAT
+
+# export BAZ=baz should be ignored.
+
+"""
+
+RESULT = {
+    'FOO': 'foo',
+    'BAR': 'bar bar bar',
+    'CPPFLAGS': '-std=c++11 -frtti -fno-strict-aliasing -DWOMBAT',
+    }
+
+BAD_ENV = ENV + """
+This line isn't right.
+NO SPACES IN NAMES="valid value"
+"""
+
+class test_ReadEnvFile(TestCase):
+  def test_read_json(self):
+    self.assertEqual(read_env_file(JSON), RESULT)
+
+  def test_read_env(self):
+    self.assertEqual(read_env_file(ENV), RESULT)
+
+  def test_read_env_error(self):
+    errors = []
+    self.assertEqual(read_env_file(BAD_ENV, errors.append), RESULT)
+    self.assertEqual(errors, [
+        "WARNING: Didn't understand the following environment file lines:",
+        "11. >>> This line isn't right.",
+        '12. >>> NO SPACES IN NAMES="valid value"'])