Respect SDKROOT in lldb Makefile
- If it is not defined (empty) everything remain as it was before.

Patch (1 of 2) by Adam Strzelecki!
SymbolVendor/ELF is actually referenced by default by lib/Makefile on all
platforms and called in lldb.cpp while it is built only on some, excluding OSX.
There is no reason to not build it then by default on all platforms.

This fixes build on OSX using llvm configure & make scripts.

Patch (2 of 2) by Adam Strzelecki!