fix Issue 13149 - released libphobos2.a is build with PIC code

- Happens because the libphobos2.so was build with a recursive
  target and PIC=1. This PIC=1 enabled -fPIC for the recursive
  libdruntime.a build.

- fixed by building $(LIBSO) non-recursively
Merge pull request #2345 from MartinNowak/fix13149

fix Issue 13149 - released libphobos2.a is build with PIC code