Bug 1816737 - silence warning spam in (third-party) zlib code r=glandium

zlib still uses K&R-style function declarations. As of clang 15, these
are warned about by default, as they've been removed in the latest draft
of C2x; this results in thousands(?) of lines of warnings when building
zlib in-tree.

An issue has been filed at https://github.com/madler/zlib/issues/633,
but the author has stated that they are unwilling to change this until
C2x has actually been published.

For now, just silence those warnings when building zlib.

Differential Revision: https://phabricator.services.mozilla.com/D169842

modules/zlib/src/moz.build

@@ -37,3 +37,7 @@ SOURCES += [
 
 # We allow warnings for third-party code that can be updated from upstream.
 AllowCompilerWarnings()
+
+# ... most warnings, anyway. https://github.com/madler/zlib/issues/633
+if CONFIG['CC_TYPE'] in ('clang', 'clang-cl'):
+    CFLAGS += ["-Wno-deprecated-non-prototype"]