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/Terminal.py

@@ -0,0 +1,36 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import sys
+
+from beast.platform.Platform import PLATFORM
+
+# See https://stackoverflow.com/questions/7445658/how-to-detect-if-the-console-does-support-ansi-escape-codes-in-python
+CAN_CHANGE_COLOR = (
+    hasattr(sys.stderr, "isatty")
+    and sys.stderr.isatty()
+    and not PLATFORM.startswith('Windows'))
+
+
+# See https://en.wikipedia.org/wiki/ANSI_escape_code
+RED = 91
+GREEN = 92
+BLUE = 94
+
+def add_mode(text, *modes):
+    if CAN_CHANGE_COLOR:
+        modes = ';'.join(str(m) for m in modes)
+        return '\033[%sm%s\033[0m' % (modes, text)
+    else:
+        return text
+
+def blue(text):
+    return add_mode(text, BLUE)
+
+def green(text):
+    return add_mode(text, GREEN)
+
+def red(text):
+    return add_mode(text, RED)
+
+def warn(text, print=print):
+    print('%s %s' % (red('WARNING:'), text))