How can I build ghostscript with an alternative zlib? - ghostscript

Question

I'm using ghostscript to generate rather large PDF files, and profiling has lead me to believe that a lot of time is spent compressing data.
For whatever reason, the ghostscript source tree ships with a copy of the zlib 1.2.11 source code, which is then compiled into the resulting gs executable.
I would like to benchmark other zlib implementations, notably Cloudflare's and possibly Intel's.
In ghostscript's Makefile.in, there's an interesting section:
# Define the directory where the zlib sources are stored.
# See zlib.mak for more information.
SHARE_ZLIB=#SHARE_ZLIB#
ZSRCDIR=#ZLIBDIR#
#ZLIB_NAME=gz
ZLIB_NAME=z
ZLIB_CFLAGS=#ZLIBCFLAGS#
And looking in base/zlib.mak:
# makefile for zlib library code.
# Users of this makefile must define the following:
# GSSRCDIR - the GS library source directory
# ZSRCDIR - the source directory
# ZGENDIR - the generated intermediate file directory
# ZOBJDIR - the object directory
# SHARE_ZLIB - 0 to compile zlib, 1 to share
# ZLIB_NAME - if SHARE_ZLIB=1, the name of the shared library
# ZAUXDIR - the directory for auxiliary objects.
So, in theory, one should just compile zlib elsewhere, get an .so file (perhaps?), set SHARE_ZLIB to 1 and ZLIB_name to /foo/bar/zlib_cloudflare/libz.so and everything should be good. Except it doesn't work, and there's zero documentation.