xerces: make sure to use the cross-compiled curl

When curl support is enabled in xerces, and a host curl is available
on the system, it might try to use it, so this patch adds --with-curl=
to the configure options to explicitly indicate where is the curl
library to use.

Should fix:

  http://autobuild.buildroot.org/results/c3c/c3c6be45d0f764fb931a46dbf1509ccf2916ecbf/



  (We were not able to reproduce specifically this problem, but
  another similar problem, where curl was detected, but considered as
  non-working because of the compilation failure of a test program.)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>