configure: fix a regression in PCRE v1 detection

configure: fix a regression in PCRE v1 detection

Change the check for PCRE v1 to disable the --with-libpcre1 option if
the pcre_version() function can't be found in the pcre library. I
unintentionally changed this in my 94da9193a6 ("grep: add support for
PCRE v2", 2017-06-01) while renaming moving some variables.

The intent of this check ever since it was added in
a119f91e57 ("configure: Check for libpcre", 2011-05-09) is to
second-guess the user and turn off an explicitly provided
--with-libpcre if the library can't be found.

I don't think that behavior makes any sense, we shouldn't be
second-guessing the user with an auto-detection, but changing that
needs a bigger refactoring of this script, and only has marginal
benefits. So let's fix it to work as it was intended to work again.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>