Allow building with dynamic libssl

[iczero]

The cmake build will always statically link libssl and libcrypto. In some cases, it may be preferable to dynamically link them. This adds -DSSL_DYNAMIC=1 which will build libxquic dynamically linked to libssl and libcrypto and also disable libxquic-static.

[CLAassistant]

All committers have signed the CLA.

[iczero]

Rebased and fixed the format problem.

Please note the "not found" message is no longer required. If the library is not found, cmake will print the following:

-- Could NOT find SSL (missing: SSL_LIBRARY_STATIC CRYPTO_LIBRARY_STATIC) 
CMake Error at CMakeLists.txt:63 (message):
  ssl module not found

[Kulsk]

LGTM