[PATCH] Do not link the dyndd runtime library against libdl on FreeBSD

[Ed Maste]

================
Comment at: lib/tsan/dd/CMakeLists.txt:27
@@ +26,3 @@
+# There is no libdl on FreeBSD.
+if(NOT ${CMAKE_SYSTEM_NAME} MATCHES FreeBSD)
+  list(APPEND DD_LINKLIBS dl)
----------------
There are a couple of examples of optionally adding -ldl without relying on the system name, in other llvm projects:

`lib/Support/CMakeLists.txt`:
```
    if( HAVE_LIBDL )
      set(system_libs ${system_libs} ${CMAKE_DL_LIBS})
```

`projects/libcxxabi/src/Unwind/CMakeLists.txt`:
```
append_if(libraries LIBCXXABI_HAS_DL_LIB dl)
```

http://reviews.llvm.org/D5207