On linux and darwin, ensure we can resolve dlopen et al

Fixes #634

Author: flavorjones

ext/sqlite3/extconf.rb

@@ -97,6 +97,10 @@ def configure_packaged_libraries
 
           ldflags.each { |ldflag| append_ldflags(ldflag) }
 
+          unless windows? || have_func("dlopen")
+            append_ldflags("-ldl")
+          end
+
           append_cppflags("-DUSING_PACKAGED_LIBRARIES")
           append_cppflags("-DUSING_PRECOMPILED_LIBRARIES") if cross_build?
         end
@@ -194,6 +198,10 @@ def darwin?
         RbConfig::CONFIG["target_os"].include?("darwin")
       end
 
+      def windows?
+        RbConfig::CONFIG["target_os"].match?(/mingw|mswin/)
+      end
+
       def print_help
         print(<<~TEXT)
           USAGE: ruby #{$PROGRAM_NAME} [options]