build: fix cmake warning with newer CMake

Hoist the compatibility macros out a level and re-use them when adding link
dependencies.  Silences a warning from CMake.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@213469 91177308-0d34-0410-b5e6-96231b3b80d8
linux process: silence GCC switch coverage warning

Add missing entry for eExecMessage message type to silence GCC switch coverage
warning.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@213470 91177308-0d34-0410-b5e6-96231b3b80d8