Disable colored output if stdout or stderr is not a TTY

This will make it easy to spread colors everywhere without worrying
about garbage test baselines. It will also clean build logs.

Change-Id: I976b74fe1712ea8e5fbfd4476f4814ae375448eb
TN: minor

langkit/utils.py

@@ -10,6 +10,7 @@ from copy import copy
 from itertools import takewhile
 
 import inspect
+import sys
 
 
 class StructEq(object):
@@ -95,6 +96,22 @@ class Colors:
     WARNING = YELLOW
     FAIL = RED
 
+    @classmethod
+    def disable_colors(cls):
+        """
+        Reset color codes to empty strings.
+        """
+        for name in dir(cls):
+            value = getattr(cls, name)
+            if (all(c.isalpha() for c in name) and
+                    name.upper() == name and
+                    isinstance(value, str)):
+                setattr(cls, name, '')
+
+
+if not sys.stdout.isatty() or not sys.stderr.isatty():
+    Colors.disable_colors()
+
 
 def col(msg, color):
     return "{0}{1}{2}".format(color, msg, Colors.ENDC)