Merge pull request #17606 from matoro/master

Ruby tooling: respect user toolchain overrides

src/ruby/ext/grpc/extconf.rb

@@ -24,10 +24,18 @@ grpc_config = ENV['GRPC_CONFIG'] || 'opt'
 
 ENV['MACOSX_DEPLOYMENT_TARGET'] = '10.7'
 
-ENV['AR'] = RbConfig::CONFIG['AR'] + ' rcs'
-ENV['CC'] = RbConfig::CONFIG['CC']
-ENV['CXX'] = RbConfig::CONFIG['CXX']
-ENV['LD'] = ENV['CC']
+if ENV['AR'].nil? || ENV['AR'].size == 0
+    ENV['AR'] = RbConfig::CONFIG['AR'] + ' rcs'
+end
+if ENV['CC'].nil? || ENV['CC'].size == 0
+    ENV['CC'] = RbConfig::CONFIG['CC']
+end
+if ENV['CXX'].nil? || ENV['CXX'].size == 0
+    ENV['CXX'] = RbConfig::CONFIG['CXX']
+end
+if ENV['LD'].nil? || ENV['LD'].size == 0
+    ENV['LD'] = ENV['CC']
+end
 
 ENV['AR'] = 'libtool -o' if RUBY_PLATFORM =~ /darwin/