configure: fix reference to possibly undefined $PKG_CONFIG_PATH

configure: fix reference to possibly undefined $PKG_CONFIG_PATH

In case zlib not found by pkg-config(1) the pkg-config information
is resolved by attempting to print ZLIB_VERSION from from zlib
installation if it exists anyway.

If above done successfully compat/zlib.pc is written for forthcoming
pkg-config execution.

Since `set -u` is in effect (since 124a67e96, 2016-05-06),
expanding unset $PKG_CONFIG_PATH (would have) failed whenever tried.

Now it is changed to set as "$PKG_CONFIG_PATH:compat" if PKG_CONFIG_PATH
is set and is non-empty string, plain "compat" otherwise.