These changes allow AEL to better check ${} constructs within $[...], that are concat...

These changes allow AEL to better check ${} constructs within $[...], that are concatenated with text.

I modified and added rules in ast_expr2.fl to better handle
the concatenations.

I added some default routines to ast_expr2.y so the standalone would
compile. It also looks like I haven't run this thru bison since 2.1, so
it's good to get this updated.

The Makefile has comments added now for check_expr2 and check_expr to
explain what they are for, and how to run them.

The testexpr2s stuff has been removed, in favor of check_expr2.

expr2.testinput has been updated to include the two expressions
that inspired these changes (from mcnobody on #asterisk this morning)
The regression has been run and all looks well.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@179807 65c4cc65-6c06-0410-ace0-fbb531ad65f3