diff --git a/waflib/ConfigSet.py b/waflib/ConfigSet.py
index 32c28a5d..de0c3aee 100644
--- a/waflib/ConfigSet.py
+++ b/waflib/ConfigSet.py
@@ -211,9 +211,9 @@ class ConfigSet(object):
 
 		The value must be a list or a tuple
 		"""
-		current_value = self._get_list_value_for_modification(var)
 		if isinstance(val, str): # if there were string everywhere we could optimize this
 			val = [val]
+		current_value = self._get_list_value_for_modification(var)
 		current_value.extend(val)
 
 	def prepend_value(self, var, val):
diff --git a/waflib/Tools/ccroot.py b/waflib/Tools/ccroot.py
index 0dc0c7ba..d4ef200c 100644
--- a/waflib/Tools/ccroot.py
+++ b/waflib/Tools/ccroot.py
@@ -409,19 +409,17 @@ def propagate_uselib_vars(self):
 	"""
 	_vars = self.get_uselib_vars()
 	env = self.env
+	app = env.append_value
+	for var in _vars:
+		y = var.lower()
+		val = getattr(self, y, [])
+		if val:
+			app(var, val)
 
-	for x in _vars:
-		y = x.lower()
-		env.append_value(x, self.to_list(getattr(self, y, [])))
-
-	for x in self.features:
-		for var in _vars:
-			compvar = '%s_%s' % (var, x)
-			env.append_value(var, env[compvar])
-
-	for x in self.to_list(getattr(self, 'uselib', [])):
-		for v in _vars:
-			env.append_value(v, env[v + '_' + x])
+		for x in self.features + self.uselib:
+			val = env['%s_%s' % (var, x)]
+			if val:
+				app(var, val)
 
 # ============ the code above must not know anything about import libs ==========
 
diff --git a/waflib/extras/compat15.py b/waflib/extras/compat15.py
index efbadd1b..4cfc4994 100644
--- a/waflib/extras/compat15.py
+++ b/waflib/extras/compat15.py
@@ -213,6 +213,7 @@ def apply_uselib_local(self):
 	names = self.to_list(getattr(self, 'uselib_local', []))
 	get = self.bld.get_tgen_by_name
 	seen = set([])
+	seen_uselib = set([])
 	tmp = Utils.deque(names) # consume a copy of the list of names
 	if tmp:
 		if Logs.verbose:
@@ -259,9 +260,11 @@ def apply_uselib_local(self):
 
 		# add ancestors uselib too - but only propagate those that have no staticlib defined
 		for v in self.to_list(getattr(y, 'uselib', [])):
-			if not env['STLIB_' + v]:
-				if not v in self.uselib:
-					self.uselib.insert(0, v)
+			if v not in seen_uselib:
+				seen_uselib.add(v)
+				if not env['STLIB_' + v]:
+					if not v in self.uselib:
+						self.uselib.insert(0, v)
 
 		# if the library task generator provides 'export_includes', add to the include path
 		# the export_includes must be a list of paths relative to the other library