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/GetEnvironment.py

@@ -0,0 +1,59 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+from beast.platform import Platform
+from beast.util import Dict
+from beast.env import Tags
+
+_DEFAULTS = {
+    '': {
+        'BOOST_HOME': None,
+        'CC': 'gcc',
+        'CCFLAGS': None,
+        'CFLAGS': None,
+        'CXX': 'g++',
+        'CPPFLAGS': '-std=c++11 -frtti -fno-strict-aliasing',
+        'CPPPATH': [],
+        'LIBPATH': [],
+        'LIBS': [],
+        'LINKFLAGS': '',
+    },
+
+    'Darwin': {
+        'CC': 'clang',
+        'CXX': 'clang++',
+        'CPPFLAGS': '-x c++ -stdlib=libc++ -std=c++11 -frtti',
+        'LINKFLAGS': '-stdlib=libc++',
+     },
+
+    'FreeBSD': {
+        'CC': 'gcc46',
+        'CXX': 'g++46',
+        'CCFLAGS': '-Wl,-rpath=/usr/local/lib/gcc46',
+        'LINKFLAGS': '-Wl,-rpath=/usr/local/lib/gcc46',
+        'LIBS': ['kvm'],
+    },
+
+    # TODO: specific flags for Windows, Linux platforms.
+}
+
+TAGS = {
+    'debug': {
+        'CPPFLAGS': '-g -DDEBUG'
+        },
+
+    'optimize': {
+        'CPPFLAGS': '-O3',
+        },
+
+    'nooptimize': {
+        'CPPFLAGS': '-O0',
+        }
+    }
+
+def get_environment(arguments):
+    tags = Tags.get_tags(arguments)
+    env = Dict.compose_prefix_dicts(Platform.PLATFORM, _DEFAULTS)
+    for tag in tags or []:
+        for k, v in TAGS.get(tag, {}).items():
+            env[k] = '%s %s' % (env[k], v)
+    return env