preproc.c: Context-through single macros expansion is deprecated

preproc.c: Context-through single macros expansion is deprecated

For now we inform users about their sources need to be
updated and also since _all_ context case are legit
for single macros only we split lookup into two phases:

1) Lookup in active context, which is perfectly valid
2) Lookup in external contexts, which will be deprecated soon.

If (2) happens we yield warning.

A typical testcase is
---
  %macro one 0
  %push
    %$a:
    %assign %$b 12
      %push
        mov eax, %$a
        mov eax, %$b  ; hit -- context through
      %pop
    %pop
  %endmacro
  one
---

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>