ext: export only Init_sqlite3_native

This works around new Darwin Ruby 3.2 symbol resolution issues, but
also follows best practices around shipping shared libraries.

Note that we remove the `-flat_namespace` hack because the latest
rake-compiler-docker images have the changes from
rake-compiler/rake-compiler-dock#94

ext/sqlite3/extconf.rb

@@ -55,6 +55,7 @@ def configure_packaged_libraries
             additional_cflags = [
               "-fPIC", # needed for linking the static library into a shared library
               "-O2", # see https://github.com/sparklemotion/sqlite3-ruby/issues/335 for some benchmarks
+              "-fvisibility=hidden", # see https://github.com/rake-compiler/rake-compiler-dock/issues/87
             ]
             env["CFLAGS"] = [env["CFLAGS"], additional_cflags].flatten.join(" ")
             recipe.configure_options += env.select { |k,v| ENV_ALLOWLIST.include?(k) }
@@ -81,14 +82,9 @@ def configure_packaged_libraries
           abort_pkg_config("xpopen") unless $?.success?
           ldflags = ldflags.split
 
-          if needs_darwin_linker_hack
-            ldflags.delete("-lsqlite3")
-            ldflags.prepend("-Wl,-flat_namespace", "-Wl,-hidden-lsqlite3")
-          else
-            # see https://github.com/flavorjones/mini_portile/issues/118
-            "-L#{lib_path}".tap do |lib_path_flag|
-              ldflags.prepend(lib_path_flag) unless ldflags.include?(lib_path_flag)
-            end
+          # see https://github.com/flavorjones/mini_portile/issues/118
+          "-L#{lib_path}".tap do |lib_path_flag|
+            ldflags.prepend(lib_path_flag) unless ldflags.include?(lib_path_flag)
           end
 
           ldflags.each { |ldflag| append_ldflags(ldflag) }
@@ -100,6 +96,8 @@ def configure_extension
           append_cppflags("-DTAINTING_SUPPORT")
         end
 
+        append_cflags("-fvisibility=hidden") # see https://github.com/rake-compiler/rake-compiler-dock/issues/87
+
         if find_header("sqlite3.h")
           # noop
         elsif sqlcipher? && find_header("sqlcipher/sqlite3.h")
@@ -177,13 +175,6 @@ def download
         minimal_recipe.download
       end
 
-      def needs_darwin_linker_hack
-        # See https://github.com/rake-compiler/rake-compiler-dock/issues/87 for more info.
-        cross_build? &&
-          darwin? &&
-          RbConfig::CONFIG["ruby_version"] >= "3.2"
-      end
-
       def darwin?
         RbConfig::CONFIG["target_os"].include?("darwin")
       end

ext/sqlite3/sqlite3.c

@@ -127,6 +127,7 @@ void init_sqlite3_constants()
 #endif
 }
 
+RUBY_FUNC_EXPORTED
 void Init_sqlite3_native()
 {
   /*