No bug - add a comment explaining why zlib's sources can't be unified. DONTBUILD

modules/zlib/src/moz.build

@@ -18,6 +18,7 @@ else:
     FINAL_LIBRARY = 'zlib'
     HostLibrary('hostzlib')
 
+# These files can't be unified because zlib's headers don't use include guards.
 SOURCES += [
     'adler32.c',
     'compress.c',