Support `if !COND', `else COND', `end COND'.

Support `if !COND', `else COND', `end COND'.
* automake.texi (Conditionals): Document it.
* automake.in ($WHITE_PATTERN, $MACRO_PATTERN, $BOGUS_MACRO_PATTERN)
($GNITS_VERSION_PATTERN, $INCLUDE_PATTERN): Use `d' and `s'.
($IF_PATTERN, $ELSE_PATTERN, $ENDIF_PATTERN): Likewise, and accept
a leading `!' before the condition.
(&handle_options): Use `d'.
(&cond_stack_if, &cond_stack_else, &&cond_stack_endif): New.
(&read_am_file, &file_contents_internal): Use them.
(&transform): No longer substitute `%!COND%', forcing the use of
`! %?COND%'.
* ansi2knr.am, lex.am, tags.am, texinfos.am, yacc.am: Adjust.