zlib: disable call to ldconfig

By default, zlib installation procedure calls ldconfig, which takes
time uselessly. ldconfig is only useful if you install libraries on
the host (in directories listed in /etc/ld.so.conf, or in /usr/lib or
/lib), so calling it after installing libraries in $(STAGING_DIR),
$(TARGET_DIR) or $(HOST_DIR) is just a lenghty no-op.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>