host-pkgconf: use --static option for static builds

Detect when BR2_PREFER_STATIC_LIBS is selected and modify the host
pkg-config wrapper to append the --static option in that case.

Fixes:
   http://autobuild.buildroot.net/results/161/161446dde7e8e774773eb2b34fd555f5ac22dd02/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>