fix homebrew installation on some machines

Formula/SwagGen.rb

@@ -8,6 +8,33 @@ class Swaggen < Formula
   depends_on :xcode
 
   def install
+
+    # libxml2 has to be included in ISYSTEM_PATH for building one of
+    # dependencies. It didn't happen automatically before Xcode 9.3
+    # so homebrew patched environment variable to get it work.
+    #
+    # That works fine when you have just Xcode installed, but there
+    # is also CLT. If it is also installed, ISYSTEM_PATH has
+    # a reference to CLT libxml2 AND a reference to Xcode default
+    # toolchain libxml2. That causes build failure with "module redeclared"
+    # error. So if both Xcode and CLT are installed one reference
+    # has to be removed.
+    #
+    # It's a bug of homebrew but before it's fixed, it's easier
+    # to provide in-place workaround for now.
+    # Please remove this once homebrew is patched.
+
+    # step 1: capture old value and patch environment
+    if OS::Mac::Xcode.version >= Version.new("9.3") && !OS::Mac::Xcode.without_clt? then
+      old_isystem_paths = ENV["HOMEBREW_ISYSTEM_PATHS"]
+      ENV["HOMEBREW_ISYSTEM_PATHS"] = old_isystem_paths.gsub("/usr/include/libxml2", "")
+    end
+
+    # step 2: usual build
     system "make", "install", "PREFIX=#{prefix}"
+
+    # step 3: restoring environment to pristine state
+    ENV["HOMEBREW_ISYSTEM_PATHS"] = old_isystem_paths if defined? old_isystem_paths
+
   end
 end