* etags.c (PHP_functions): New function by Diez B. Roggisch,

* etags.c (PHP_functions): New function by Diez B. Roggisch,
heavily adapted by me, for parsing PHP.
(LOOKING_AT): New macro.
(Perl_functions, Python_functions, PHP_functions)
(Scheme_functions, Texinfo_nodes): Use it.
(Perl_functions): Use strneq.
(prolog_pred): Renamed to prolog_pr.
(prolog_pr): Recognise Prolog rules (thanks to Geert Kloosterman)
in addition to predicates.
[ETAGS_REGEXPS] [!HAVE_CONFIG_H] [__CYGWIN__]: Prevent
unmodified compile, as Cygwin's regex.h is incompatible with us
(thanks to Markus Hoenicka).
[!HAVE_CONFIG_H] [!__STDC__]: #define const as the empty string.