Not upstreamable hack. Gentoo installs /usr/${libdir}/libz.so
linker script that redirects to /${libdir}/libz.so.1

Let's use ELF file directly. It also requires guile to support
direct loading and enforces >=guile-3.
--- a/zlib/config.scm.in
+++ b/zlib/config.scm.in
@@ -20,4 +20,4 @@
   #:export (%libz))
 
 (define %libz
-  "@LIBZ_LIBDIR@/libz")
+  "@LIBZ_LIBDIR@/libz.so.1")