Remove eval-when-compile.

Fixes byte-compiler error of the form:

Error: Symbol's value as variable is void: php-extra-constants
Add (require 'etags) as etag functions are used.

Fixes byte-compiler warnings:

Warning: the following functions are not known to be
    defined: tags-verify-table, etags-tags-completion-table

etags.el is part of GNU Emacs so should be safe to include.