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/platform/Platform.py

@@ -0,0 +1,27 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import platform
+
+def _get_platform_string():
+    system = platform.system()
+    parts = [system]
+    linux = system == 'Linux'
+    if linux:
+        flavor, version, _ = platform.linux_distribution()
+        # Arch still has issues with the platform module
+        parts[0] = flavor.capitalize() or 'Archlinux'
+        parts.extend(version.split('.'))
+    elif system == 'Darwin':
+        ten, major, minor = platform.mac_ver()[0].split('.')
+        parts.extend([ten, major, minor])
+    elif system == 'Windows':
+        release, version, csd, ptype = platform.win32_ver()
+        parts.extend([release, version, csd, ptype])
+    elif system == 'FreeBSD':
+        # No other variables to pass with FreeBSD that Python provides and I could find
+        pass
+    else:
+        raise Exception("Don't understand how to build for platform " + system)
+    return '.'.join(parts), linux
+
+PLATFORM, IS_LINUX = _get_platform_string()