qt5-qmake: remove /usr/include for -isystem

gcc6 does not like it, if -isystem /usr/include is last (or at all?)
in the list of -isystem paths. Just removing this path from the
QMAKE_DEFAULT_INCDIRS solves problems when compiling packages
depending on qt5.

srcpkgs/qt5/template

@@ -1,7 +1,7 @@
 # Template file for 'qt5'
 pkgname=qt5
 version=5.6.2
-revision=8
+revision=9
 wrksrc="qt-everywhere-opensource-src-${version}"
 homepage="http://qt.io/"
 short_desc="A cross-platform application and UI framework (QT5)"
@@ -773,5 +773,8 @@ qt5-qmake_package() {
 		vmove usr/lib/qt5/bin/qmake
 		vmove usr/lib/qt5/mkspecs
 		vmove usr/bin/qmake-qt5
+		# Remove /usr/include from the QMAKE_DEFAULT_INCDIRS
+		sed -i ${PKGDESTDIR}/usr/lib/qt5/mkspecs/qconfig.pri \
+			-e 's;/usr/include$;;'
 	}
 }