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/Boost.py

@@ -0,0 +1,61 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import os
+import re
+
+ROOT_ENV_VARIABLE = 'BOOST_ROOT'
+MINIMUM_VERSION = 1, 55, 0
+VERSION_FILE = 'boost', 'version.hpp'
+LIBRARY_PATH_SEGMENT = 'stage', 'lib'
+VERSION_MATCHER = re.compile(r'#define\s+BOOST_VERSION\s+(\d+)')
+
+CANT_OPEN_VERSION_FILE_ERROR = """Unable to open boost version file %s.
+You have set the environment variable BOOST_ROOT to be %s.
+Please check to make sure that this points to a valid installation of boost."""
+
+CANT_UNDERSTAND_VERSION_ERROR = (
+    "Didn't understand version string '%s' from file %s'")
+VERSION_TOO_OLD_ERROR = ('Your version of boost, %s, is older than the minimum '
+                         'required, %s.')
+
+def _text(major, minor, release):
+    return '%d.%02d.%02d' % (major, minor, release)
+
+def _raw_boost_path():
+    try:
+      path = os.environ[ROOT_ENV_VARIABLE]
+      if path:
+        return os.path.normpath(path)
+    except KeyError:
+        pass
+    raise KeyError('%s environment variable is not set.' % ROOT_ENV_VARIABLE)
+
+def _get_version_number(path):
+    version_file = os.path.join(path, *VERSION_FILE)
+    try:
+        with open(version_file) as f:
+            for line in f:
+                match = VERSION_MATCHER.match(line)
+                if match:
+                    version = match.group(1)
+                    try:
+                        return int(version)
+                    except ValueError:
+                        raise Exception(CANT_UNDERSTAND_VERSION_ERROR %
+                                        (version, version_file))
+    except IOError:
+        raise Exception(CANT_OPEN_VERSION_FILE_ERROR % (version_file, path))
+
+def _validate_version(v):
+    version = v // 100000, (v // 100) % 100, v % 100
+    if version < MINIMUM_VERSION:
+        raise Exception(VERSION_TOO_OLD_ERROR % (
+    _text(*version), _text(*MINIMUM_VERSION)))
+
+def _boost_path():
+    path = _raw_boost_path()
+    _validate_version(_get_version_number(path))
+    return path
+
+CPPPATH = _boost_path()
+LIBPATH = os.path.join(CPPPATH, *LIBRARY_PATH_SEGMENT)