- Declare token YYEOF to be able to support older bison versions
as well

- Be sure to have a usable bison installed at build time

- Add upstream patches
* bash52-021
There is an off-by-one error that causes command substitutions to fail when
they appear in a word expansion inside a here-document.
* bash52-020
The parser did not allow `time' to appear as the first reserved word in a
command substitution.
* bash52-019
There are some cases where the shell reaped a background (asynchronous) job
and would incorrectly try to set the terminal's process group back to the
shell's. In these cases it never set the terminal process group to that
jobs's process group initially, so resetting it is incorrect.
* bash52-018
There are two problems with returning tokens to yyparse() when the shell
encounters a syntax error or when it reads EOF.
When reading a WORD token, the parser has to return the correct value to
yyparse. Previous versions returned a value < 0, which the bash parser
translated into YYERRCODE for bison, and in newer versions of bison, the
appropriate reset actions didn't happen. We should return YYUNDEF, which
bison uses for `invalid token'. Since we can return a token < 0 for both
invalid tokens and EOF, the bash tokenizer needs to differentiate between
those two cases.
* bash52-017
In certain cases, using the `.' builtin in a subshell would optimize away
the rest of the commands in the subshell.