Linux: Relax interdependency between freetype, libpng, and zlib for unvendored builds

This restriction was added to fix #7373 back then, which was a symbol conflict
between FreeType's bundled copy of gzip/zlib, and distro packages.

But we also unbundled FreeType's zlib in #69395 so this is no longer an issue.

On the other hand recent issues pointed out that using system-provided icu4c
or harfbuzz can cause issues (#91401, #100301). We still allow it for now but
raise a warning.

core/SCsub

@@ -99,6 +99,8 @@ if env["builtin_zlib"]:
     env.Prepend(CPPPATH=[thirdparty_zlib_dir])
     if env.dev_build:
         env_thirdparty.Append(CPPDEFINES=["ZLIB_DEBUG"])
+        # Affects headers so it should also be defined for Godot code
+        env.Append(CPPDEFINES=["ZLIB_DEBUG"])
 
     env_thirdparty.add_source_files(thirdparty_obj, thirdparty_zlib_sources)