Stop compiling js ext sources on non-wasm platforms

Even though we early return in the extconf.rb file, the default
Makefile generator will still glob for *.c files in the ext directory
and try to compile them. This is unnecessary and can cause build
failures on non-wasm platforms.

Author: kateinoigakukun

packages/gems/js/ext/js/extconf.rb

@@ -3,6 +3,8 @@
 MakeMakefile::RbConfig ||= RbConfig
 unless MakeMakefile::RbConfig::CONFIG["platform"] =~ /wasm/
   $stderr.puts "This extension is only for WebAssembly. Creating a dummy Makefile."
+  # Explicitly set $srcs to avoid the default '*.c' globbing.
+  $srcs = []
   create_makefile("js")
   return
 end