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

Fixes [Issue 10710 – shared phobos library doesn't work on all linux distributions](https://d.puremagic.com/issues/show_bug.cgi?id=10710).
Simplifies building phobos (see [here](https://github.com/D-Programming-Language/installer/pull/24#issuecomment-30482436)).