Emscripten has an own zlib port, use it

See https://github.com/emscripten-ports/zlib

Change-Id: I0af6e2f1f1908838f940ab11706637e3407263fd

configure.ac

@@ -7420,7 +7420,11 @@ dnl and has no pkg-config for it at least on some tinderboxes,
 dnl so leaving that out for now
 dnl libo_CHECK_SYSTEM_MODULE([zlib],[ZLIB],[zlib])
 AC_MSG_CHECKING([which zlib to use])
-if test "$with_system_zlib" = "yes"; then
+if test "$_os" = "Emscripten"; then
+    # Emscripten provides its own zlib
+    AC_MSG_RESULT([Emscripten provided])
+    SYSTEM_ZLIB=TRUE
+elif test "$with_system_zlib" = "yes"; then
     AC_MSG_RESULT([external])
     SYSTEM_ZLIB=TRUE
     AC_CHECK_HEADER(zlib.h, [],

distro-configs/LibreOfficeEmscripten.conf

@@ -37,7 +37,6 @@
 --without-helppack-integration
 --without-system-dicts
 --with-theme=no
---with-system-zlib=no
 --with-system-nss=yes
 
 # Build fails when disabling these