Fix Qtopia build issues

This patch fixes two Qtopia build issues, encountered while trying to
use system implementation of zlib, freetype, jpeg and libpng :

 * The build process doesn't look in $(STAGING_DIR)/usr/include for
   includes and $(STAGING_DIR)/usr/lib. Same problem as the patch
   currently floating around adding LDFLAGS to TARGET_CONFIGURE_OPTS,
   but as Qtopia doesn't use TARGET_CONFIGURE_OPTS, we need a specific
   fix here. So we use the -I and -L options of Qtopia's configure
   script.

 * The build process doesn't use pkg-config to get the header path for
   Freetype headers (located in $(STAGING_DIR)/usr/include/freetype2
   and not directly in $(STAGING_DIR)/usr/include/). There was already
   a fix for this, consisting in adding $(FREETYPE_DIR)/include to the
   -I path of Qtopia's configure. This patch modifies this fix to use
   $(STAGING_DIR)/usr/include/freetype2 instead, which looks more
   coherent with how all the packages are built (using $(STAGING_DIR)
   as the reference to get headers and libraries).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>