fix a bug where vendor library contained in gem causes a build error with system bundled ruby

http://hipbyte.myjetbrains.com/youtrack/issue/RM-553

lib/motion/project/builder.rb

@@ -69,12 +69,6 @@ module Motion; module Project;
         bs_files.concat(vendor_project.bs_files)
       end
 
-      # Validate common build directory.
-      if !File.directory?(Builder.common_build_dir) or !File.writable?(Builder.common_build_dir)
-        $stderr.puts "Cannot write into the `#{Builder.common_build_dir}' directory, please remove or check permissions and try again."
-        exit 1
-      end
-
       # Prepare embedded and external frameworks BridgeSupport files (OSX-only).
       if config.respond_to?(:embedded_frameworks) && config.respond_to?(:external_frameworks)
         embedded_frameworks = config.embedded_frameworks.map { |x| File.expand_path(x) }
@@ -552,6 +546,13 @@ EOS
           rescue
           end
         end
+
+        # Validate common build directory.
+        if !File.directory?(dir) or !File.writable?(dir)
+          $stderr.puts "Cannot write into the `#{dir}' directory, please remove or check permissions and try again."
+          exit 1
+        end
+
         dir
       end
     end

lib/motion/project/vendor.rb

@@ -65,8 +65,7 @@ module Motion; module Project;
     def build_static(platform, opts)
       App.info 'Build', @path
       Dir.chdir(@path) do
-        build_dir = "build-#{platform}"
-
+        build_dir = build_dir(platform)
         libs = (opts.delete(:products) or Dir.glob('*.a'))
         source_files = (opts.delete(:source_files) or ['**/*.{c,m,cpp,cxx,mm,h}']).map { |pattern| Dir.glob(pattern) }.flatten
         cflags = (opts.delete(:cflags) or '')
@@ -151,7 +150,7 @@ EOS
     def build_xcode(platform, opts)
       project_dir = File.expand_path(@config.project_dir)
       Dir.chdir(@path) do
-        build_dir = "build-#{platform}"
+        build_dir = build_dir(platform)
         if !File.exist?(build_dir) or Dir.glob('**/*').any? { |x| File.mtime(x) > File.mtime(build_dir) }
           FileUtils.mkdir_p build_dir
 
@@ -229,6 +228,16 @@ EOS
 
     private
 
+    def build_dir(platform)
+      @build_dir ||= begin
+        path = "build-#{platform}"
+        unless File.writable?(Dir.pwd)
+          path = File.join(Builder.common_build_dir, @path, path)
+        end
+        path
+      end
+    end
+
     def gen_method(prefix)
       method = "#{prefix}_#{@type.to_s}".intern
       raise "Invalid vendor project type: #{@type}" unless respond_to?(method)