package/git: add optional support for pcre2

package/git: add optional support for pcre2

Renamed --with-libpcre to --with-libpcre1. Currently --with-libpcre
activates pcre1 support but this can change in the future to pcre2:
https://github.com/git/git/blob/df7fd961a9d9ba60840ffc0868d36cc3db2aec74/configure.ac#L258

Please note that we cannot use --with-/--without because it will lead
to an error reported by configure, for example

    --with-libpcre1 --without-libpcre2

will produce

configure: error: Only supply one of --with-libpcre1 or --with-libpcre2!

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>