extconf.rb: locate zstd when using homebrew

Fix compilation issues on my system.

ext/mysql2/extconf.rb

@@ -66,6 +66,12 @@ def add_ssl_defines(header)
   $LDFLAGS << " -L#{openssl_location}/lib" if openssl_location
 end
 
+if RUBY_PLATFORM =~ /darwin/ && system('command -v brew')
+  zstd_location = `brew --prefix zstd`.strip
+
+  $LIBPATH << "#{zstd_location}/lib" unless zstd_location.empty?
+end
+
 ### Find MySQL client library
 
 # borrowed from mysqlplus