Re: [PATCH 22/29] grep: change the internal PCRE macro names to be PCRE1

> Change the internal USE_LIBPCRE define, & build options flag to use a
> naming convention ending in PCRE1, without changing the long-standing
> USE_LIBPCRE Makefile flag which enables this code.
> 
> This is for preparation for libpcre2 support where having things like
> USE_LIBPCRE and USE_LIBPCRE2 in any more places than we absolutely
> need to for backwards compatibility with old Makefile arguments would
> be confusing.
> 
> In some ways it would be better to change everything that now uses
> USE_LIBPCRE to use USE_LIBPCRE1, and to make specifying
> USE_LIBPCRE (or --with-pcre) an error. This would impose a one-time
> burden on packagers of git to s/USE_LIBPCRE/USE_LIBPCRE1/ in their
> build scripts.
> 
> However I'd like to leave the door open to making
> USE_LIBPCRE=YesPlease eventually mean USE_LIBPCRE2=YesPlease,
> i.e. once PCRE v2 is ubiquitous enough that it makes sense to make it
> the default.