tools/configure: fix pkg-config install path for FreeBSD

tools/configure: fix pkg-config install path for FreeBSD

pkg-config from FreeBSD ports doesn't have ${prefix}/share/pkgconfig in the
default search path, fix this by having a PKG_INSTALLDIR variable that can
be changed on a per-OS basis.

It would be best to use PKG_INSTALLDIR as defined by the pkg.m4 macro, but
sadly this also reports a wrong value on FreeBSD (${libdir}/pkgconfig, which
expands to /usr/local/lib/pkgconfig by default, and is also _not_ part of
the default pkg-config search path).

This patch should not change the behavior for Linux installs.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reported-by: Alexander Nusov <alexander.nusov@nfvexpress.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>