Redo the _whooollleee_ build system

Following the tutorials on http://www.ostricher.com/tag/scons/, I rebuilt the
build system so that SConscript files are *much* easier to write.

site_scons/site_tools/lib.py

@@ -0,0 +1,59 @@
+# lib.py
+# Eryn Wells <eryn@erynwells.me>
+
+'''
+SCons builder for a lib directory.
+'''
+
+import SCons.Errors
+import SCons.Script
+
+
+def _process_lib_dir(env, lib, src_dir=None, out_dir=None, inc_dir=None):
+    if not src_dir:
+        src_dir = env.lib_root.Dir(lib)
+    if not src_dir.isdir():
+        err = 'Invalid library source directory: {}'.format(src_dir)
+        env.log_error(err)
+        raise SCons.Errors.UserError(err)
+    if not out_dir:
+        out_dir = env.build_root.Dir('lib').Dir(lib)
+    if not inc_dir:
+        include_dir = src_dir.Dir('include')
+        if include_dir.isdir():
+            inc_dir = [include_dir]
+    env.Append(CPPPATH=inc_dir)
+    exports = {'Library': env.Library,
+               'StaticLibrary': env.StaticLibrary,
+               'SharedLibrary': env.SharedLibrary}
+    SCons.Script._SConscript.GlobalDict.update(exports)
+    out = env.SConscript(src_dir.File('SConscript'),
+                         {'env': env.Clone()},
+                         variant_dir=out_dir,
+                         exports=exports)
+    return out
+
+
+def _build_library(env, lib_func):
+    original_builder = lib_func
+
+    def builder(env, lib_name, sources, *args, **kwargs):
+        lib = original_builder(lib_name, sources, *args, **kwargs)
+        env.register_lib(lib_name, lib)
+        return lib
+
+    return builder
+
+#
+# SCons tool interface
+#
+
+def generate(env):
+    env.SetDefault(LIBS={})
+    env.AddMethod(_process_lib_dir, 'LibDir')
+    env.AddMethod(_build_library(env, env.Library), 'Library')
+    env.AddMethod(_build_library(env, env.StaticLibrary), 'StaticLibrary')
+    env.AddMethod(_build_library(env, env.SharedLibrary), 'SharedLibrary')
+
+def exists(env):
+    return True