In gentoo qt5 already requires c++11 features:

diff --git a/src/xxdiff.pro b/src/xxdiff.pro
index 32b106b..16fcd1f 100644
--- a/src/xxdiff.pro
+++ b/src/xxdiff.pro
@@ -113,3 +113,3 @@ irix-n32:QMAKE_CFLAGS_RELEASE += -OPT:Olimit=4000
 # auto_ptr deprecated in C++11, removed in C++17
-linux: QMAKE_CXXFLAGS += -std=c++03
+linux: QMAKE_CXXFLAGS += -std=c++11