Uses/php.mk: Use EGREP instead of GREP when parsing config.h

Using ${GREP} was failing when grep was built with the WITHOUT_GNU_GREP_COMPAT
knob in src.conf. According to POSIX, | only works for alternation in extended
regular expressions (grep -E), although GNU grep disregards the difference.

The new syntax with ${EGREP} and no escaping of special characters works with
both BSD grep (without WITHOUT_GNU_GREP_COMPAT) as well as GNU grep.

Approved by: mat
Differential Revision: https://reviews.freebsd.org/D8663