depends: build qt in c++17 mode

depends/packages/qt.mk

@@ -29,7 +29,7 @@ $(package)_config_opts_release = -release
 $(package)_config_opts_release += -silent
 $(package)_config_opts_debug = -debug
 $(package)_config_opts += -bindir $(build_prefix)/bin
-$(package)_config_opts += -c++std c++11
+$(package)_config_opts += -c++std c++1z
 $(package)_config_opts += -confirm-license
 $(package)_config_opts += -hostprefix $(build_prefix)
 $(package)_config_opts += -no-compile-examples