Move the last instance of AST_LIBS to the only place it is used,

Move the last instance of AST_LIBS to the only place it is used,
namely main/Makefile .

I am unclear where decisions on the build environment (CFLAGS,
LDFLAGS, LIBS and so on) should be made - right now they are
split here and there.

As a first step in cleaning up this situation, i am trying to at
least collect all instances of each variable in one place.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@88767 65c4cc65-6c06-0410-ace0-fbb531ad65f3