qt5-build.eclass: switch to using system proxy settings by default

But only in 5.6 and later, we don't want to touch stable 5.5.1.

Upstream is switching to this new setting in 5.8, see qtbase
commit 6d9f648d0ed591a0afba018adfbb089871f67b38.

eclass/qt5-build.eclass

@@ -594,9 +594,6 @@ qt5_base_configure() {
 		# print verbose information about each configure test
 		-verbose
 
-		# obsolete flag, does nothing
-		#-nis
-
 		# always enable iconv support
 		-iconv
 
@@ -650,8 +647,9 @@ qt5_base_configure() {
 		# disable gstreamer by default, override in qtmultimedia
 		-no-gstreamer
 
-		# use upstream default
-		#-no-system-proxies
+		# respect system proxies by default: it's the most natural
+		# setting, and it'll become the new upstream default in 5.8
+		$([[ ${QT5_MINOR_VERSION} -ge 6 ]] && echo -system-proxies)
 
 		# do not build with -Werror
 		-no-warnings-are-errors