fix Issue 10710 - phobos depends on versioned libcurl

- create a libcurl_stub.so for linking

- set it's soname to libcurl.so.4 so that phobos
  gets a DT_NEEDED libcurl.so.4 entry

- the runtime linker will load libcurl.so and resolve
  all undefined references

- disable pragma(lib, "curl") because it doesn't work
  for archives anyways and additionally links against
  the real libcurl