[depends] allow NO_OPENSSL=1

Summary: OpenSSL is not needed when BIP70 is disabled so this allows skipping the compilation during cross-building, which is useful for testing if a crossbuild issue is specifically caused by this lib.

Test Plan:

make -C depends HOST="x86_64-linux-gnu" NO_OPENSSL=1
make -C depends HOST="x86_64-apple-darwin" NO_OPENSSL=1
make -C depends HOST="x86_64-w64-mingw32" NO_OPENSSL=1

Check that the gitian builds still work with the qt.mk changes.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15570