qt: speed up qmake build

qmake is built during the execution of Qt ./configure script, so it is
built just with a normal make, not taking advantage of parallel
compilation. Passing MAKEFLAGS=-j$(BR2_JLEVEL) allows to speed up the
qmake compilation process quite a bit.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/qt/qt.mk

@@ -468,7 +468,7 @@ define QT_CONFIGURE_CMDS
 		PKG_CONFIG_SYSROOT_DIR="$(STAGING_DIR)" \
 		PKG_CONFIG="$(PKG_CONFIG_HOST_BINARY)" \
 		PKG_CONFIG_PATH="$(STAGING_DIR)/usr/lib/pkgconfig:$(PKG_CONFIG_PATH)" \
-		./configure \
+		MAKEFLAGS="$(MAKEFLAGS) -j$(BR2_JLEVEL)" ./configure \
 		$(if $(VERBOSE),-verbose,-silent) \
 		-force-pkg-config \
 		$(QT_CONFIGURE_OPTS) \