diff --git a/Library/Homebrew/extend/os/mac/extend/ENV/std.rb b/Library/Homebrew/extend/os/mac/extend/ENV/std.rb
index 049fa6adce..2df379cde5 100644
--- a/Library/Homebrew/extend/os/mac/extend/ENV/std.rb
+++ b/Library/Homebrew/extend/os/mac/extend/ENV/std.rb
@@ -97,12 +97,14 @@ module Stdenv
   end
 
   # Some configure scripts won't find libxml2 without help
+  # This is a no-op with macOS SDK 10.15.4 and later
   def libxml2
-    if !MacOS.sdk_path_if_needed
+    sdk = MacOS.sdk_path_if_needed
+    if !sdk
       append "CPPFLAGS", "-I/usr/include/libxml2"
-    else
+    elsif !(sdk/"usr/include/libxml").directory?
       # Use the includes form the sdk
-      append "CPPFLAGS", "-I#{MacOS.sdk_path}/usr/include/libxml2"
+      append "CPPFLAGS", "-I#{sdk}/usr/include/libxml2"
     end
   end
 
diff --git a/Library/Homebrew/extend/os/mac/extend/ENV/super.rb b/Library/Homebrew/extend/os/mac/extend/ENV/super.rb
index acbdd982fd..104aa9861f 100644
--- a/Library/Homebrew/extend/os/mac/extend/ENV/super.rb
+++ b/Library/Homebrew/extend/os/mac/extend/ENV/super.rb
@@ -42,9 +42,17 @@ module Superenv
     paths
   end
 
+  # @private
+  def libxml2_include_needed?
+    return false if deps.any? { |d| d.name == "libxml2" }
+    return false if Pathname("#{self["HOMEBREW_SDKROOT"]}/usr/include/libxml").directory?
+
+    true
+  end
+
   def homebrew_extra_isystem_paths
     paths = []
-    paths << "#{self["HOMEBREW_SDKROOT"]}/usr/include/libxml2" unless deps.any? { |d| d.name == "libxml2" }
+    paths << "#{self["HOMEBREW_SDKROOT"]}/usr/include/libxml2" if libxml2_include_needed?
     paths << "#{self["HOMEBREW_SDKROOT"]}/usr/include/apache2" if MacOS::Xcode.without_clt?
     paths << MacOS::X11.include.to_s << "#{MacOS::X11.include}/freetype2" if x11?
     paths << "#{self["HOMEBREW_SDKROOT"]}/System/Library/Frameworks/OpenGL.framework/Versions/Current/Headers"
@@ -64,7 +72,7 @@ module Superenv
 
   def homebrew_extra_cmake_include_paths
     paths = []
-    paths << "#{self["HOMEBREW_SDKROOT"]}/usr/include/libxml2" unless deps.any? { |d| d.name == "libxml2" }
+    paths << "#{self["HOMEBREW_SDKROOT"]}/usr/include/libxml2" if libxml2_include_needed?
     paths << "#{self["HOMEBREW_SDKROOT"]}/usr/include/apache2" if MacOS::Xcode.without_clt?
     paths << MacOS::X11.include.to_s << "#{MacOS::X11.include}/freetype2" if x11?
     paths << "#{self["HOMEBREW_SDKROOT"]}/System/Library/Frameworks/OpenGL.framework/Versions/Current/Headers"