cmake: enable static linking of OpenSSL if BUILD_STATIC is set

Currently, the BUILD_STATIC and BUILD_STATIC_WITH_BUNDLED_LIBS cmake
options don't enable static linking of the OpenSSL library so we have
to set OPENSSL_USE_STATIC_LIBS explicitly. Let's enable static linking
of OpenSSL by default because we enable it anyway in all our official
builds.

NO_DOC=build
NO_TEST=build
NO_CHANGELOG=build