Use pkgconfig to detect curl first, then fallback to curl-config.

Change-Id: I3666c4d0298a76062a24ad8ae1c319f3480a9290
Reviewed-on: https://gerrit.libreoffice.org/7643
Reviewed-by: Khaled Hosny <khaledhosny@eglug.org>
Tested-by: Khaled Hosny <khaledhosny@eglug.org>