[pan2] Fix build with libc++

[Petr Kovář <pmkovar src gnome org>]

commit 44ffc85e90af75046ca6dc94268b4b9045b5c6d5
Author: Petr Kovar <pknbe volny cz>
Date:   Fri Mar 18 21:22:51 2016 +0100

    Fix build with libc++
    
    Thank you goes to Detlef Graef, David Evans, and Thomas Klausner.

 pan/data-impl/groups.cc |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/pan/data-impl/groups.cc b/pan/data-impl/groups.cc
index c76d5a1..9f1cfe5 100644
--- a/pan/data-impl/groups.cc
+++ b/pan/data-impl/groups.cc
@@ -81,7 +81,15 @@ namespace
   }
 }
 
+// detect std::lib
+#include <ciso646>
+#ifdef _LIBCPP_VERSION
+// using libc++
+#include <algorithm>
+#else
+// using libstdc++
 #include <ext/algorithm>
+#endif
 
 void
 DataImpl :: load_newsrc (const Quark       & server,