Avoid clobbering LDFLAGS and CPPFLAGS if in a nix prepared environment (#526)

* Avoid nuking LDFLAGS if using nix

* Actual code change

* Use ENV.key? to check for nix env var

* Consistent use of ENV.key?

---------

Co-authored-by: Maciej Mensfeld <[email protected]>

Author: secobarbital

ext/Rakefile

@@ -16,11 +16,11 @@ task :default => :clean do
     require "mini_portile2"
     recipe = MiniPortile.new("librdkafka", Rdkafka::LIBRDKAFKA_VERSION)
 
-    # Use default homebrew openssl if we're on mac and the directory exists
-    # and each of flags is not empty
-    if recipe.host&.include?("darwin") && system("which brew &> /dev/null") && Dir.exist?("#{homebrew_prefix = %x(brew --prefix openssl).strip}")
-      ENV["CPPFLAGS"] = "-I#{homebrew_prefix}/include" unless ENV["CPPFLAGS"]
-      ENV["LDFLAGS"] = "-L#{homebrew_prefix}/lib" unless ENV["LDFLAGS"]
+    # Use default homebrew openssl if we're on mac and the directory exists, is not using nix-prepared libraries
+    # and each of flags is not already set
+    if recipe.host&.include?("darwin") && system("which brew &> /dev/null") && Dir.exist?("#{homebrew_prefix = %x(brew --prefix openssl).strip}") && ENV.key?("NIX_LDFLAGS")
+      ENV["CPPFLAGS"] = "-I#{homebrew_prefix}/include" unless ENV.key?("CPPFLAGS")
+      ENV["LDFLAGS"] = "-L#{homebrew_prefix}/lib" unless ENV.key?("LDFLAGS")
     end
 
     releases = File.expand_path(File.join(File.dirname(__FILE__), '../dist'))