Fix ruby_abi_version symbol error on macOS

Replace static exports.txt with dynamic export file selection based on
Ruby version, following grpc's approach.

Changes:
- Remove exports.txt
- Add ext-export.txt (for release versions)
- Add ext-export-with-ruby-abi-version.txt (for dev versions)
- Update extconf.rb to dynamically select export file based on Ruby version

This fixes the 'Undefined symbols: _ruby_abi_version' linker error
that occurs when building the gem.

Related:
- https://bugs.ruby-lang.org/issues/19289
- grpc/grpc#31970
- #117

Author: SpringMT

ext/zstdruby/exports.txt

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

ext/zstdruby/ext-export-with-ruby-abi-version.txt

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

ext/zstdruby/ext-export.txt

@@ -2,12 +2,38 @@
 
 have_func('rb_gc_mark_movable')
 
+# Check if ruby_abi_version symbol is required
+# Based on grpc's approach: https://github.com/grpc/grpc/blob/master/src/ruby/ext/grpc/extconf.rb
+def have_ruby_abi_version?
+  # Only development/preview versions need the symbol
+  return false if RUBY_PATCHLEVEL >= 0
+  
+  # Ruby 3.2+ development versions require ruby_abi_version
+  major, minor = RUBY_VERSION.split('.').map(&:to_i)
+  if major > 3 || (major == 3 && minor >= 2)
+    puts "Ruby version #{RUBY_VERSION} >= 3.2. Using ruby_abi_version symbol."
+    return true
+  else
+    puts "Ruby version #{RUBY_VERSION} < 3.2. Not using ruby_abi_version symbol."
+    return false
+  end
+end
+
+# Determine which export file to use based on Ruby version
+def ext_export_filename
+  name = 'ext-export'
+  name += '-with-ruby-abi-version' if have_ruby_abi_version?
+  name
+end
+
 $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__)}"
+  ext_export_file = File.join(__dir__, "#{ext_export_filename}.txt")
+  $LDFLAGS += " -exported_symbols_list #{ext_export_file}"
+  puts "Using export file: #{ext_export_file}"
 end
 
 Dir.chdir File.expand_path('..', __FILE__) do