Always set sysroot for Xcode-only systems

Fixes Homebrew/homebrew#33431.

Library/ENV/4.3/cc

@@ -33,7 +33,7 @@ class Cmd
     @args = args.freeze
     @brewfix = ENV['HOMEBREW_PREFIX']
     @brewtmp = ENV['HOMEBREW_TEMP']
-    @sysroot = ENV['HOMEBREW_SYSROOT']
+    @sysroot = ENV['HOMEBREW_SDKROOT']
   end
 
   def mode
@@ -93,7 +93,7 @@ class Cmd
       if tool == "ld"
         args << "-syslibroot" << sysroot
       else
-        args << "--sysroot=#{sysroot}"
+        args << "-isysroot" << sysroot << "--sysroot=#{sysroot}"
       end
     end
 

Library/Homebrew/extend/ENV/super.rb

@@ -46,7 +46,7 @@ module Superenv
     self['HOMEBREW_BREW_FILE'] = HOMEBREW_BREW_FILE.to_s
     self['HOMEBREW_PREFIX'] = HOMEBREW_PREFIX.to_s
     self['HOMEBREW_TEMP'] = HOMEBREW_TEMP.to_s
-    self['HOMEBREW_SDKROOT'] = "#{MacOS.sdk_path}" if MacOS::Xcode.without_clt?
+    self['HOMEBREW_SDKROOT'] = effective_sysroot
     self['HOMEBREW_OPTFLAGS'] = determine_optflags
     self['HOMEBREW_ARCHFLAGS'] = ''
     self['CMAKE_PREFIX_PATH'] = determine_cmake_prefix_path
@@ -59,12 +59,6 @@ module Superenv
     self["HOMEBREW_INCLUDE_PATHS"] = determine_include_paths
     self["HOMEBREW_LIBRARY_PATHS"] = determine_library_paths
 
-    # On 10.9 the developer tools honor the correct sysroot by default.
-    # On 10.7 and 10.8 we need to set it ourselves.
-    if MacOS::Xcode.without_clt? && (MacOS.version <= "10.8" || compiler != :clang)
-      self["HOMEBREW_SYSROOT"] = effective_sysroot
-    end
-
     # On 10.9, the tools in /usr/bin proxy to the active developer directory.
     # This means we can use them for any combination of CLT and Xcode.
     self["HOMEBREW_PREFER_CLT_PROXIES"] = "1" if MacOS.version >= "10.9"
@@ -96,7 +90,7 @@ module Superenv
   end
 
   def effective_sysroot
-    if MacOS::Xcode.without_clt? then MacOS.sdk_path.to_s else "" end
+    MacOS::Xcode.without_clt? ? MacOS.sdk_path.to_s : nil
   end
 
   def determine_cxx