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

Fixes #634

Author: flavorjones

ext/sqlite3/extconf.rb

@@ -194,6 +194,10 @@ 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]

patches/001-dl-and-pthread-flags.patch

@@ -0,0 +1,17 @@
+commit d9eae6b7 (HEAD -> master, origin/master, origin/HEAD)
+Author: stephan <[email protected]>
+Date:   2025-07-04 17:20:37 +0000
+
+    Propagate the -ldl and -lpthread flags, if needed, into sqlite3.pc, per request in [forum:44a58c807353162f | forum post 44a58c807353162f].
+    
+    FossilOrigin-Name: 64f5f14322349b47451b8cac03bf8cd6f1ae45a8822e7f1ddee3d0b265047501
+
+--- a/sqlite3.pc.in
++++ b/sqlite3.pc.in
+@@ -9,5 +9,5 @@ Name: SQLite
+ Description: SQL database engine
+ Version: @PACKAGE_VERSION@
+ Libs: -L${libdir} -lsqlite3
+-Libs.private: @LDFLAGS_MATH@ @LDFLAGS_ZLIB@ @LDFLAGS_ICU@
++Libs.private: @LDFLAGS_MATH@ @LDFLAGS_ZLIB@ @LDFLAGS_DLOPEN@ @LDFLAGS_PTHREAD@ @LDFLAGS_ICU@
+ Cflags: -I${includedir}