Qt5 requires C++11, make it explicit for compilers where it's not the default

Makefile.in

@@ -17,7 +17,7 @@ incpath = -I. -I$(PREFIX)/include
 libpath = -L$(PREFIX)/lib
 
 CFLAGS = $(warn) $(dbg) $(opt) $(incpath) -fPIC $(add_cflags) -MMD
-CXXFLAGS = $(warn) $(dbg) $(opt) $(incpath) -fPIC `pkg-config --cflags Qt5Core Qt5Gui Qt5Widgets` \
+CXXFLAGS = -std=c++11 $(warn) $(dbg) $(opt) $(incpath) -fPIC `pkg-config --cflags Qt5Core Qt5Gui Qt5Widgets` \
 		 $(add_cflags) -MMD
 LDFLAGS = $(libpath) `pkg-config --libs Qt5Core Qt5Gui Qt5Widgets` -lspnav -lX11 \
 		  $(add_ldflags)