configure: removed $zlib_(cflags|ldflags) from compat code build

configure: removed $zlib_(cflags|ldflags) from compat code build

When pkg-config does not find configure, a compat version of the
zlib.pc is created. In creation of that configure attempted to
read values of $zlib_cflags and $zlib_ldflags. In the usual case
those were undefined, and with `set -a` now in the beginning of
configure, configure broke.

Even if $zlib_cflags and $zlib_ldflags had values which were used
to create zlib.pc, the values were overwritten (with static content)
a few lines later in next pkg-config --cflags and --libs run. These
values would not be different and probably useless -- the following
boild would probably fail.

But instead of using those, CPATH and LIBRARY_PATH environment
variables can be used successfully (both while configuring and
building).