Allow setting sysroots to a CLT SDK

Library/Homebrew/extend/os/mac/extend/ENV/std.rb

@@ -27,7 +27,7 @@ module Stdenv
 
     append_path "ACLOCAL_PATH", "#{MacOS::X11.share}/aclocal"
 
-    if MacOS::XQuartz.provided_by_apple? && !MacOS::CLT.installed?
+    if MacOS::XQuartz.provided_by_apple? && MacOS.sdk_path
       append_path "CMAKE_PREFIX_PATH", "#{MacOS.sdk_path}/usr/X11"
     end
 
@@ -93,7 +93,7 @@ module Stdenv
     delete("CPATH")
     remove "LDFLAGS", "-L#{HOMEBREW_PREFIX}/lib"
 
-    return unless (sdk = MacOS.sdk_path(version)) && !MacOS::CLT.installed?
+    return unless (sdk = MacOS.sdk_path(version))
     delete("SDKROOT")
     remove_from_cflags "-isysroot #{sdk}"
     remove "CPPFLAGS", "-isysroot #{sdk}"
@@ -115,7 +115,7 @@ module Stdenv
     self["CPATH"] = "#{HOMEBREW_PREFIX}/include"
     prepend "LDFLAGS", "-L#{HOMEBREW_PREFIX}/lib"
 
-    return unless (sdk = MacOS.sdk_path(version)) && !MacOS::CLT.installed?
+    return unless (sdk = MacOS.sdk_path(version))
     # Extra setup to support Xcode 4.3+ without CLT.
     self["SDKROOT"] = sdk
     # Tell clang/gcc where system include's are:
@@ -132,7 +132,7 @@ module Stdenv
 
   # Some configure scripts won't find libxml2 without help
   def libxml2
-    if MacOS::CLT.installed?
+    if !MacOS.sdk_path
       append "CPPFLAGS", "-I/usr/include/libxml2"
     else
       # Use the includes form the sdk

Library/Homebrew/extend/os/mac/extend/ENV/super.rb

@@ -58,7 +58,7 @@ module Superenv
   def homebrew_extra_library_paths
     paths = []
     if compiler == :llvm_clang
-      if MacOS::CLT.installed?
+      if !MacOS.sdk_path
         paths << "/usr/lib"
       else
         paths << "#{MacOS.sdk_path}/usr/lib"
@@ -102,7 +102,7 @@ module Superenv
   end
 
   def effective_sysroot
-    MacOS.sdk_path.to_s if MacOS::Xcode.without_clt?
+    MacOS.sdk_path&.to_s
   end
 
   def set_x11_env_if_installed
@@ -113,7 +113,7 @@ module Superenv
   def setup_build_environment(formula = nil)
     generic_setup_build_environment(formula)
     self["HOMEBREW_SDKROOT"] = effective_sysroot
-    self["SDKROOT"] = MacOS.sdk_path if MacOS::Xcode.without_clt?
+    self["SDKROOT"] = MacOS.sdk_path if MacOS.sdk_path
 
     # Filter out symbols known not to be defined since GNU Autotools can't
     # reliably figure this out with Xcode 8 and above.