Fix static library build

[mertemba]

The libsocketConfig.cmake.in assumes that the library target is called socket++, but the static library is currently called socket++_int. Thus, using the static build of libsocket with FetchContent is currently broken.

This commit changes the static library target name to the same as the shared library. The drawback is that now BUILD_SHARED_LIBS and BUILD_STATIC_LIBS cannot be enabled both at the same time, because it would lead to a name collision.