libnss: fix build failures

Such as
http://autobuild.buildroot.net/results/2ca1d135269a67201e6b4d995ce8fbe94db1ed3f



The mozilla build system passes all of the different ways of CFLAGS
(OPTIMIZER, XCFLAGS, OS_CFLAGS) even when building host tools
(nsintall).
The best way around this without patching libnss is just building the
host tools first without any CFLAGS and then going on with the rest.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>