configure: Fix --disable-pcre2

Previously it would result in neither pcre library being detected:

    checking for getnameinfo... (cached) yes
    configure: WARNING: Ignoring pcre2 even if it's available
    test: =: unexpected operator
    Enabling support for client-specific tags.
    checking for zlibVersion in -lz... (cached) yes
    Enabling compression support.
    test: =: unexpected operator
    test: =: unexpected operator
    configure: error: Detected neither pcre2 nor pcre library.