qt5: avoid LDFLAGS leaking into mkspecs

Instead of patching mkspecs files set the desired
CFLAGS, CXXFLAGS and LDFLAGS through QMAKE_... variables
when configuring.

See #3112

srcpkgs/qt5/template

@@ -1,7 +1,7 @@
 # Template file for 'qt5'
 pkgname=qt5
 version=5.5.1
-revision=2
+revision=3
 wrksrc="qt-everywhere-opensource-src-${version}"
 homepage="http://qt.io/"
 short_desc="A cross-platform application and UI framework (QT5)"
@@ -26,9 +26,6 @@ makedepends="
 depends="qtchooser"
 
 pre_configure() {
-	sed -i "s|-O2|${CXXFLAGS}|" qtbase/mkspecs/common/{g++,gcc}-base.conf
-	sed -i "/^QMAKE_LFLAGS\s/s|+=|+= ${LDFLAGS}|g" qtbase/mkspecs/common/gcc-base.conf
-
 	# Compatibility functions res_ninit() and res_nclose() for musl libc
 	cp ${FILESDIR}/resolv_compat.h ${wrksrc}/qtwebengine/src/3rdparty/chromium/net/dns
 	case "$XBPS_TARGET_MACHINE" in
@@ -71,6 +68,9 @@ do_configure() {
 		*)	opts+=" -reduce-relocations"
 			;;
 	esac
+	QMAKE_CFLAGS="${CFLAGS}" \
+	QMAKE_CXXFLAGS="${CXXFLAGS}" \
+	QMAKE_LFLAGS="${LDFLAGS}" \
 		./configure ${opts}
 }
 do_build() {