[lldb/Host] Use Host/Config.h entries instead of a global define.

As suggested by Pavel in a code review:

> Can we replace this (and maybe python too, while at it) with a
> Host/Config.h entry? A global definition means that one has to
> recompile everything when these change in any way, whereas in
> practice only a handful of files need this..

Differential revision: https://reviews.llvm.org/D71280

lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

@@ -6,11 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifdef LLDB_DISABLE_PYTHON
+#include "lldb/Host/Config.h"
 
-// Python is disabled in this build
-
-#else
+#ifndef LLDB_DISABLE_PYTHON
 
 // LLDB Python header must be included first
 #include "lldb-python.h"