superenv: fix cpp shim

This sets cpp to use the compiler as cpp, and works with scripts which
pass values to cpp on stdin as well as with file arguments.

Closes Homebrew/homebrew#22155.

Library/ENV/4.3/cc

@@ -6,6 +6,7 @@
 #TODO create mechanism to specify build effects like %w{-O0 -O4 vanilla-arg-parsing sdk=10.6} etc.
 
 require "#{File.dirname __FILE__}/../libsuperenv"
+require 'io/wait'
 require 'set'
 
 def cccfg? flags
@@ -55,8 +56,7 @@ class Cmd
   def tool
     @tool ||= case @arg0
     when 'ld' then 'ld'
-    when 'cpp' then 'cpp'
-    when 'cc', 'c99', 'c89'
+    when 'cc', 'cpp', 'c99', 'c89'
       # Ideally we would run `cx9`, however these tools are POSIX compliant
       # and don't support many flags. We need -isystem for instance, but also
       # reliability is generally much higher if we just get clang/gcc to do
@@ -90,7 +90,8 @@ class Cmd
     end if nclt?
     case mode
     when :cpp
-      %w{-E} + args + cppflags
+      args = %w{-E} + args + cppflags
+      STDIN.ready? ? args + %w{-} : args
     when :ld
       ldflags + args
     when :cc