When openldap-server is built within an environment where ICU has been
previously built, the package will pick up this unintended dependency and
fail with the following error:

    Package openldap-server is missing dependencies for the following libraries:
    libicudata.so.55
    libicuuc.so.55
    Makefile:148: recipe for target '.../openldap-server_2.4.43-2_mips_34kc.ipk' failed

Since the ICU libraries are extremely big (~12MB) we cannot simply depend on
them so add the necessary autoconfig cache variable to let the ICU presence
test fail in order to inhibit linking against those libraries.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>