Compile C files by C compiler when C++ files mixed.

ref #3267 #3470

By this commit, mruby do not use C++ ABI mode unless
you specify explicitly. It compiles C files by C
compilers, with C++ exception enabled when it sees
C++ files in your configured mrbgems.

I haven't tried visualcpp, so please submit an issue
if you see any problem with C++ gems on Windows.

doc/guides/compile.md

@@ -242,9 +242,19 @@ conf.enable_bintest
 ### C++ ABI
 
 mruby can use C++ exception to raise exception internally.
-It is called C++ ABI mode.
 By using C++ exception it can release C++ stack object correctly.
-Whenever you mix C++ code C++ ABI mode would be enabled automatically.
+
+There are two levels of C++ exception handling. The one is
+C++ exception enabled (but still C files are compiled by C
+compiler), and the other is C++ ABI mode where all files are
+compiled by C++ compiler.
+
+When you mix C++ code, C++ exception would be enabled automatically.
+If you need to enable C++ exception explicitly add the following:
+```ruby
+conf.enable_cxx_exception
+```
+
 If you need to enable C++ ABI mode explicitly add the following:
 ```ruby
 conf.enable_cxx_abi
@@ -252,8 +262,10 @@ conf.enable_cxx_abi
 
 #### C++ exception disabling.
 
+
 If you need to force C++ exception disable
-(For example using a compiler option to disable C++ exception)
+(For example using a compiler option to disable C++ exception),
+but still want to use C++ ABI mode,
 add following:
 ```ruby
 conf.disable_cxx_exception

tasks/mruby_build.rake

@@ -113,13 +113,23 @@ module MRuby
       @cxx_exception_disabled = true
     end
 
+    def enable_cxx_exception
+      @cxx_exception_disabled = false
+      compilers.each { |c|
+        c.defines += %w(MRB_ENABLE_CXX_EXCEPTION)
+        c.flags << c.cxx_exception_flag
+      }
+    end
+
     def cxx_abi_enabled?
       @cxx_abi_enabled
     end
 
     def enable_cxx_abi
-      return if @cxx_exception_disabled or @cxx_abi_enabled
-      compilers.each { |c| c.defines += %w(MRB_ENABLE_CXX_EXCEPTION) }
+      return if @cxx_abi_enabled
+      unless @cxx_exception_disabled
+        enable_cxx_exception
+      end
       compilers.each { |c| c.flags << c.cxx_compile_flag }
       linker.command = cxx.command if toolchains.find { |v| v == 'gcc' }
       @cxx_abi_enabled = true

tasks/mruby_build_commands.rake

@@ -41,7 +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
+    attr_accessor :cxx_compile_flag, :cxx_exception_flag
 
     def initialize(build, source_exts=[])
       super(build)

tasks/mruby_build_gem.rake

@@ -38,7 +38,7 @@ module MRuby
       cxx_srcs = ['src', 'test', 'tools'].map do |subdir|
         Dir.glob("#{Gem.current.dir}/#{subdir}/*.{cpp,cxx,cc}")
       end.flatten
-      enable_cxx_abi unless cxx_srcs.empty?
+      enable_cxx_exception unless cxx_srcs.empty?
 
       Gem.current
     end

tasks/toolchains/gcc.rake

@@ -7,6 +7,7 @@ MRuby::Toolchain.new(:gcc) do |conf, _params|
     cc.option_define = '-D%s'
     cc.compile_options = '%{flags} -MMD -o %{outfile} -c %{infile}'
     cc.cxx_compile_flag = '-x c++ -std=c++03'
+    cc.cxx_exception_flag = '-fexceptions'
   end
 
   [conf.cxx].each do |cxx|
@@ -17,6 +18,7 @@ MRuby::Toolchain.new(:gcc) do |conf, _params|
     cxx.option_define = '-D%s'
     cxx.compile_options = '%{flags} -MMD -o %{outfile} -c %{infile}'
     cxx.cxx_compile_flag = '-x c++ -std=c++03'
+    cxx.cxx_exception_flag = '-fexceptions'
   end
 
   conf.linker do |linker|

tasks/toolchains/visualcpp.rake

@@ -8,6 +8,7 @@ MRuby::Toolchain.new(:visualcpp) do |conf, _params|
     cc.option_define = '/D%s'
     cc.compile_options = "%{flags} /Fo%{outfile} %{infile}"
     cc.cxx_compile_flag = '/TP'
+    cc.cxx_exception_flag = '/EHs'
   end
 
   conf.cxx do |cxx|
@@ -18,6 +19,7 @@ MRuby::Toolchain.new(:visualcpp) do |conf, _params|
     cxx.option_define = '/D%s'
     cxx.compile_options = "%{flags} /Fo%{outfile} %{infile}"
     cxx.cxx_compile_flag = '/TP'
+    cxx.cxx_exception_flag = '/EHs'
   end
 
   conf.linker do |linker|