Fix symbol collision with other native gems using libzstd

- Hide all ZSTD symbols by default using -fvisibility=hidden
- Use exported_symbols_list on macOS to explicitly control exported symbols
- Only export Init_zstdruby function required for Ruby extension initialization
- Add RUBY_FUNC_EXPORTED macro to Init_zstdruby function

This resolves symbol collision issues when using zstd-ruby alongside other
gems that dynamically link to system libzstd (e.g., rdkafka-ruby).

Fixes #102

Author: SpringMT

ext/zstdruby/exports.txt

@@ -0,0 +1 @@
+_Init_zstdruby 

ext/zstdruby/extconf.rb

@@ -2,9 +2,14 @@
 
 have_func('rb_gc_mark_movable')
 
-$CFLAGS = '-I. -O3 -std=c99 -DZSTD_STATIC_LINKING_ONLY -DZSTD_MULTITHREAD -pthread -DDEBUGLEVEL=0'
+$CFLAGS = '-I. -O3 -std=c99 -DZSTD_STATIC_LINKING_ONLY -DZSTD_MULTITHREAD -pthread -DDEBUGLEVEL=0 -fvisibility=hidden -DZSTDLIB_VISIBLE=\'__attribute__((visibility("hidden")))\' -DZSTDLIB_HIDDEN=\'__attribute__((visibility("hidden")))\''
 $CPPFLAGS += " -fdeclspec" if CONFIG['CXX'] =~ /clang/
 
+# macOS specific: Use exported_symbols_list to control symbol visibility
+if RUBY_PLATFORM =~ /darwin/
+  $LDFLAGS += " -exported_symbols_list #{File.expand_path('exports.txt', __dir__)}"
+end
+
 Dir.chdir File.expand_path('..', __FILE__) do
   $srcs = Dir['**/*.c', '**/*.S']
 

ext/zstdruby/main.c

@@ -8,7 +8,7 @@ void zstd_ruby_skippable_frame_init(void);
 void zstd_ruby_streaming_compress_init(void);
 void zstd_ruby_streaming_decompress_init(void);
 
-void
+RUBY_FUNC_EXPORTED void
 Init_zstdruby(void)
 {
 #ifdef HAVE_RB_EXT_RACTOR_SAFE