...where external/libxmlsec/ExternalProject_xmlsec.mk uses win32/configure.js
instead of configure.  But that inconsistency generally got hidden on Windows by
xmlsecurity/inc/pch/precompiled_xsec_xmlsec.hxx including the raw xmlsec include
files instead of xmlsecurity/xmlsec-wrapper.h, so only gets noticed when using
--disable-pch (as is needed when building with clang-cl).

TODO: Don't know how to fix bin/update_pch so that precompiled_xsec_xmlsec.hxx
doesn't get broken again on the next update.

Change-Id: If385066a1e0f949d943aab7e9a5e969fbf0d0794