bind: don't lookup zlib.h in host headers

bind: don't lookup zlib.h in host headers

configure.in looks in host headers for zlib.h, unless given a headers
directory as --with-zlib parameter.

Note: a bug in the zlib.h header lookup logic causes configure.in to add
-l$(STAGING_DIR)/usr/include/include, and -L$(STAGING_DIR)/usr/include/lib.
But this does not affect us.

Fixes:
http://autobuild.buildroot.net/results/e96/e96a36c4da3c3be4b79a27af75a70bb8955c31a9/
http://autobuild.buildroot.net/results/e0b/e0bd7df5c19c7c65ce0009b7c2b4d4104a5c3109/
http://autobuild.buildroot.net/results/e99/e993940067f7ae841132765f91bfee7248ab125f/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>