poco: fix parallel build failure

This added patch should fix
http://autobuild.buildroot.net/results/1bf7e51ef30af9bbf5e423e80ef07212b83bdaf8/

.

CROSSENV is now required also in the install stages to prevent a rebuild using
the host native toolchain.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>