Fix for compiling mruby as C++ on Visual Studio toolchain

tasks/mruby_build.rake

@@ -120,7 +120,7 @@ module MRuby
     def enable_cxx_abi
       return if @cxx_exception_disabled or @cxx_abi_enabled
       compilers.each { |c| c.defines += %w(MRB_ENABLE_CXX_EXCEPTION) }
-      compilers.each { |c| c.flags << '-x c++'}
+      compilers.each { |c| c.flags << c.cxx_compile_flag }
       linker.command = cxx.command if toolchains.find { |v| v == 'gcc' }
       @cxx_abi_enabled = true
     end

tasks/mruby_build_commands.rake

@@ -41,6 +41,7 @@ module MRuby
   class Command::Compiler < Command
     attr_accessor :flags, :include_paths, :defines, :source_exts
     attr_accessor :compile_options, :option_define, :option_include_path, :out_ext
+    attr_accessor :cxx_compile_flag
 
     def initialize(build, source_exts=[])
       super(build)

tasks/toolchains/gcc.rake

@@ -6,6 +6,7 @@ MRuby::Toolchain.new(:gcc) do |conf, _params|
     cc.option_include_path = '-I%s'
     cc.option_define = '-D%s'
     cc.compile_options = '%{flags} -MMD -o %{outfile} -c %{infile}'
+    cc.cxx_compile_flag = '-x c++'
   end
 
   [conf.cxx].each do |cxx|
@@ -15,6 +16,7 @@ MRuby::Toolchain.new(:gcc) do |conf, _params|
     cxx.option_include_path = '-I%s'
     cxx.option_define = '-D%s'
     cxx.compile_options = '%{flags} -MMD -o %{outfile} -c %{infile}'
+    cxx.cxx_compile_flag = '-x c++'
   end
 
   conf.linker do |linker|

tasks/toolchains/visualcpp.rake

@@ -7,6 +7,7 @@ MRuby::Toolchain.new(:visualcpp) do |conf, _params|
     cc.option_include_path = '/I%s'
     cc.option_define = '/D%s'
     cc.compile_options = "%{flags} /Fo%{outfile} %{infile}"
+    cc.cxx_compile_flag = '/TP'
   end
 
   conf.cxx do |cxx|
@@ -16,6 +17,7 @@ MRuby::Toolchain.new(:visualcpp) do |conf, _params|
     cxx.option_include_path = '/I%s'
     cxx.option_define = '/D%s'
     cxx.compile_options = "%{flags} /Fo%{outfile} %{infile}"
+    cxx.cxx_compile_flag = '/TP'
   end
 
   conf.linker do |linker|