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/String.py

@@ -0,0 +1,60 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import functools
+
+from beast.util import Iter
+from beast.util.Terminal import warn
+
+def is_string(s):
+    """Is s a string? - in either Python 2.x or 3.x."""
+    return isinstance(s, (str, unicode))
+
+def stringify(item, joiner=''):
+    """If item is not a string, stringify its members and join them."""
+    try:
+        len(item)
+    except:
+        return str(item)
+    if not item or is_string(item):
+        return item or ''
+    else:
+        return joiner.join(str(i) for i in item)
+
+def single_line(line, report_errors=True, joiner='+'):
+    """Force a string to be a single line with no carriage returns, and report
+    a warning if there was more than one line."""
+    lines = line.strip().splitlines()
+    if report_errors and len(lines) > 1:
+      print('multiline result:', lines)
+    return joiner.join(lines)
+
+# Copied from
+# https://github.com/lerugray/pickett/blob/master/pickett/ParseScript.py
+def remove_comment(line):
+    """Remove trailing comments from one line."""
+    start = 0
+    while True:
+        loc = line.find('#', start)
+        if loc == -1:
+            return line.replace('\\#', '#')
+        elif not (loc and line[loc - 1] == '\\'):
+            return line[:loc].replace('\\#', '#')
+        start = loc + 1
+
+def remove_quotes(line, quote='"', print=print):
+    if not line.startswith(quote):
+        return line
+    if line.endswith(quote):
+        return line[1:-1]
+
+    warn('line started with %s but didn\'t end with one:' % quote, print)
+    print(line)
+    return line[1:]
+
+def fields_after_prefix(prefix, line):
+    line = line.strip()
+    return line.startswith(prefix) and line[len(prefix):].split()
+
+def first_fields_after_prefix(prefix, sequence):
+    condition = functools.partial(fields_after_prefix, prefix)
+    return Iter.first(condition, sequence) or []