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/util/Dict_test.py

@@ -0,0 +1,56 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+from unittest import TestCase
+
+from beast.util import Dict
+
+DICT = {
+    '': {
+        'foo': 'foo-default',
+        'bar': 'bar-default',
+      },
+
+    'Darwin': {
+        'foo': 'foo-darwin',
+        'baz': 'baz-darwin',
+      },
+
+    'Darwin.10.8': {
+        'foo': 'foo-darwin-10.8',
+        'bing': 'bing-darwin-10.8',
+      },
+  }
+
+class test_Dict(TestCase):
+    def computeMapValue(self, config, key):
+        return Dict.compose(*Dict.get_items_with_prefix(config, DICT))[key]
+
+    def assertMapValue(self, config, key, result):
+        self.assertEquals(self.computeMapValue(config, key), result)
+
+    def testDefault1(self):
+        self.assertMapValue('', 'foo', 'foo-default')
+
+    def testDefault2(self):
+        self.assertMapValue('Darwin.10.8', 'bar', 'bar-default')
+
+    def testPrefix1(self):
+        self.assertMapValue('Darwin', 'foo', 'foo-darwin')
+
+    def testPrefix2(self):
+        self.assertMapValue('Darwin.10.8', 'foo', 'foo-darwin-10.8')
+
+    def testPrefix3(self):
+        self.assertMapValue('Darwin', 'baz', 'baz-darwin')
+
+    def testPrefix4(self):
+        self.assertMapValue('Darwin.10.8', 'bing', 'bing-darwin-10.8')
+
+    def testFailure1(self):
+        self.assertRaises(KeyError, self.computeMapValue, '', 'baz')
+
+    def testFailure2(self):
+        self.assertRaises(KeyError, self.computeMapValue, '', 'bing')
+
+    def testFailure2(self):
+        self.assertRaises(KeyError, self.computeMapValue, 'Darwin', 'bing')