qt5: replace -isystem with -I in gcc-base.conf

This solves problems of the kind "#include_next <stdlib.h>" no such
file or directory. The real problem seems to be that "-isystem /usr/include"
or "-isystem ${XBPS_CROSS_BASE}/usr/include" is in the wrong place in
the list of parameters passed to gcc/g++.

Closes #5254

srcpkgs/qt5/template

@@ -1,7 +1,7 @@
 # Template file for 'qt5'
 pkgname=qt5
 version=5.7.1
-revision=1
+revision=2
 wrksrc="qt-everywhere-opensource-src-${version}"
 homepage="http://qt.io/"
 short_desc="A cross-platform application and UI framework (QT5)"
@@ -830,8 +830,8 @@ 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 ${PKGDESTDIR}/usr/lib/qt5/mkspecs/qconfig.pri \
-			-e 's;/usr/include$;;'
+		# Change -isystem to -I to avoid "#include_next <stdlib.h>" errors
+		sed -i ${PKGDESTDIR}/usr/lib/qt5/mkspecs/common/gcc-base.conf \
+			-e '/^QMAKE_CFLAGS_ISYSTEM/s;-isystem;-I;'
 	}
 }