build: add ports support for platform-specific gem sources

Add conf.ports method to specify target platform tags (e.g.,
:esp32, :rp2040, :posix). Gems with matching ports/<name>/
directories will automatically compile those sources.

Host builds auto-detect :posix or :win when not explicitly set.
Cross builds require explicit specification. Existing gems
without ports/ directories are unaffected.

Co-authored-by: Claude <noreply@anthropic.com>

lib/mruby/build.rb

@@ -81,7 +81,7 @@ module MRuby
     include LoadGems
     attr_accessor :name, :bins, :exts, :file_separator, :build_dir, :gem_clone_dir, :defines, :libdir_name
     attr_reader :products, :libmruby_core_objs, :libmruby_objs, :gems, :toolchains, :presym, :mrbc_build, :gem_dir_to_repo_url
-    attr_reader :install_excludes
+    attr_reader :install_excludes, :port_names
 
     alias libmruby libmruby_objs
 
@@ -138,6 +138,7 @@ module MRuby
         @mrbcfile_external = false
         @internal = internal
         @toolchains = []
+        @port_names = nil
         @gem_dir_to_repo_url = {}
 
         # Add lambda instead of string because libdir_name or lib may be changed by user configuration
@@ -183,6 +184,29 @@ module MRuby
       @enable_debug = true
     end
 
+    # Set target port names for this build.
+    # Gems with matching ports/<name>/ directories will compile
+    # those platform-specific sources automatically.
+    #   conf.ports :esp32
+    #   conf.ports :rp2040, :posix
+    def ports(*names)
+      @port_names = names.map { |n| n.to_s }
+    end
+
+    # Returns the effective port names for this build.
+    # If not explicitly set, auto-detects :posix or :win for host builds.
+    def effective_ports
+      return @port_names if @port_names
+      if kind_of?(MRuby::CrossBuild)
+        []
+      elsif ENV['OS'] == 'Windows_NT' ||
+            ('A'..'Z').any? { |v| Dir.exist?("#{v}:") }
+        ['win']
+      else
+        ['posix']
+      end
+    end
+
     def disable_lock
       @enable_lock = false
     end

lib/mruby/gem.rb

@@ -59,6 +59,12 @@ module MRuby
         @rbfiles = Dir.glob("#{@dir}/mrblib/**/*.rb").sort
         @objs = srcs_to_objs("src")
 
+        # Add platform-specific sources from ports/<name>/ directories
+        build.effective_ports.each do |port|
+          port_dir = "#{@dir}/ports/#{port}"
+          @objs += srcs_to_objs("ports/#{port}") if File.directory?(port_dir)
+        end
+
         @test_preload = nil # 'test/assert.rb'
         @test_args = {}
         @skip_test = false