print/pdf-tools: Add pkgconf build dependency and fix configure env

The recent import of pkgconf into the FreeBSD base system temporarily
caused a print/pdf-tools build failure and exposed two issues with the
port. First, pkgconf should be a direct build dependency. Second,
${CONFIGURE_ENV} should be passed to ./configure so that
PKG_CONFIG_LIBDIR is set correctly regardless of the pkgconf
implementation in the environment.

Sponsored by: The FreeBSD Foundation