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/File.py

@@ -0,0 +1,42 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+from beast.util import String
+
+import os
+
+LIBRARY_PATTERNS = 'lib%s.a', 'lib%s.dylib'
+
+def first_fields_after_prefix(filename, prefix):
+    with open(filename, 'r') as f:
+        return String.first_fields_after_prefix(prefix, f)
+
+def find_files_with_suffix(base, suffix):
+    for parent, _, files in os.walk(base):
+        for path in files:
+            path = os.path.join(parent, path)
+            if path.endswith(suffix):
+                yield os.path.normpath(path)
+
+def child_files(parent, files):
+    return [os.path.normpath(os.path.join(parent, f)) for f in files]
+
+def sibling_files(path, files):
+    return child_files(os.path.dirname(path), files)
+
+def replace_extension(file, ext):
+    return os.path.splitext(file)[0] + ext
+
+def validate_libraries(path, libraries):
+    bad = []
+    for lib in libraries:
+        found = False
+        for pat in LIBRARY_PATTERNS:
+            libfile = os.path.join(path, pat % lib)
+            if os.path.isfile(libfile):
+                found = True
+                break
+        if not found:
+            bad.append(libfile)
+    if bad:
+        libs = 'library' if len(bad) == 1 else 'libraries'
+        raise Exception('Missing %s: %s' % (libs, ', '.join(bad)))