build: Use flat namespace for libvirt on macOS

build: Use flat namespace for libvirt on macOS

>From ld(1):

  By default all references resolved to a dynamic library record the
  library to which they were resolved. At runtime, dyld uses that
  information to directly resolve symbols. The alternative is to use the
  -flat_namespace option.  With flat namespace, the library is not
  recorded.  At runtime, dyld will search each dynamic library in load
  order when resolving symbols. This is slower, but more like how other
  operating systems resolve symbols.

That fixes the set of tests that preload a mock library to replace
library symbols:
  qemublocktest
  qemumonitorjsontest
  viriscsitest
  virmacmaptest
  virnetserverclienttest

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>