By making it possible to use libxmlsec's mscng backend instead of the old
mscrypto one which lacks ECDSA support.

make -sr CppunitTest_xmlsecurity_signing SVL_CRYPTO_CNG=1 CPPUNIT_TEST_NAME="SigningTest::testECDSA"

passes with these changes, while it failed in the SVL_CRYPTO_CNG=1 case previously.

Change-Id: Ic23e5af11d271ed84175abe3d5ad008c7cc9e071
Reviewed-on: https://gerrit.libreoffice.org/56370Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins