webkit: several build fixes

This patch fixes different problems:

 * The autogen script of Webkit did not find autoconf, automake and
   aclocal since they are not installed in the host, but in
   $(HOST_DIR). Therefore we pass HOST_CONFIGURE_OPTS to autogen.sh so
   that the PATH is correct.

 * The autogen script was complaining that gtk-doc.make could not be
   find. As compiling gtk-doc completely is a pain (depends on
   Docbook), we just include gtk-doc.make into package/webkit/, and
   copy it to the source directory.

 * The autogen script was complaining about macros used in
   gtk-doc.make that did not exist. This is because aclocal was not
   run with $(STAGING_DIR)/usr/share/aclocal/ in the include
   path. Therefore, we fix ACLOCAL_FLAGS of Webkit's autogen script
   before running it.

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