Do not collect linker options from binary gems; close #5210

Binary gems are mrbgems that set `spec.bins` in their `mrbgem.rake`,
and usually their names are prefixed with `mruby-bin-`.

lib/mruby/gem.rb

@@ -119,6 +119,10 @@ module MRuby
         @dir.start_with?("#{MRUBY_ROOT}/mrbgems/")
       end
 
+      def bin?
+        @bins.size > 0
+      end
+
       def add_dependency(name, *requirements)
         default_gem = requirements.last.kind_of?(Hash) ? requirements.pop : nil
         requirements = ['>= 0.0.0'] if requirements.empty?
@@ -496,8 +500,10 @@ module MRuby
         end
       end
 
-      def linker_attrs
-        map{|g| g.linker.run_attrs}.transpose
+      def linker_attrs(gem=nil)
+        gems = self.reject{|g| g.bin?}  # library gems
+        gems << gem unless gem.nil?
+        gems.map{|g| g.linker.run_attrs}.transpose
       end
     end # List
   end # Gem

tasks/bin.rake

@@ -6,8 +6,8 @@ MRuby.each_target do |build|
 
   build.bins.each{|bin| build.products << define_installer_if_needed(bin)}
 
-  linker_attrs = build.gems.linker_attrs
   build.gems.each do |gem|
+    linker_attrs = build.gems.linker_attrs(gem)
     gem.bins.each do |bin|
       exe = build.exefile("#{build.build_dir}/bin/#{bin}")
       objs = Dir["#{gem.dir}/tools/#{bin}/*.{c,cpp,cxx,cc}"].map do |f|

tasks/libmruby.rake

@@ -16,14 +16,15 @@ MRuby.each_target do
     open(t.name, 'w') do |f|
       f.puts "MRUBY_CFLAGS = #{cc.all_flags}"
 
-      gem_flags = gems.map { |g| g.linker.flags }
-      gem_library_paths = gems.map { |g| g.linker.library_paths }
+      libgems = gems.reject{|g| g.bin?}
+      gem_flags = libgems.map {|g| g.linker.flags }
+      gem_library_paths = libgems.map {|g| g.linker.library_paths }
       f.puts "MRUBY_LDFLAGS = #{linker.all_flags(gem_library_paths, gem_flags)} #{linker.option_library_path % "#{build_dir}/lib"}"
 
-      gem_flags_before_libraries = gems.map { |g| g.linker.flags_before_libraries }
+      gem_flags_before_libraries = libgems.map {|g| g.linker.flags_before_libraries }
       f.puts "MRUBY_LDFLAGS_BEFORE_LIBS = #{[linker.flags_before_libraries, gem_flags_before_libraries].flatten.join(' ')}"
 
-      gem_libraries = gems.map { |g| g.linker.libraries }
+      gem_libraries = libgems.map {|g| g.linker.libraries }
       f.puts "MRUBY_LIBS = #{linker.option_library % 'mruby'} #{linker.library_flags(gem_libraries)}"
 
       f.puts "MRUBY_LIBMRUBY_PATH = #{libmruby_static}"