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/env/Print.py

@@ -0,0 +1,41 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import textwrap
+
+from beast.util import String
+from beast.util import Terminal
+
+FIELD_WIDTH = 10
+LINE_WIDTH = 69
+
+EMPTY_NAME = ' ' * FIELD_WIDTH
+
+TEXT_WRAPPER = textwrap.TextWrapper(
+    break_long_words=False,
+    break_on_hyphens=False,
+    width=LINE_WIDTH,
+)
+
+DISPLAY_EMPTY_ENVS = True
+
+def print_build_vars(name, value, same, print=print):
+    """Pretty-print values as a build configuration."""
+    name = '%s' % name.rjust(FIELD_WIDTH)
+    color = Terminal.blue if same else Terminal.green
+
+    for line in TEXT_WRAPPER.wrap(String.stringify(value, ' ')):
+        print(' '.join([name, color(line)]))
+        name = EMPTY_NAME
+
+def print_cmd_line(s, target, source, env):
+    print(EMPTY_NAME + Terminal.blue(String.stringify(target)))
+
+def print_build_config(env, original, print=print):
+    print('\nConfiguration:')
+    for name, value in env.items():
+        if value or DISPLAY_EMPTY_ENVS:
+            same = (value == original[name])
+            if not same:
+                print('"%s" != "%s"' % (value, original[name]))
+            print_build_vars(name, value, same, print=print)
+    print()