Cause error explicitly from `MRB_TRY()` with `cxx_exception`; ref #5088

`MRB_TRY()` does not work when compiled by C compiler with `cxx_exception`
configuration. We should explicitly warn.

include/mruby/throw.h

@@ -13,7 +13,9 @@
 # endif
 #endif
 
-#if defined(MRB_ENABLE_CXX_EXCEPTION) && defined(__cplusplus)
+#if defined(MRB_ENABLE_CXX_EXCEPTION)
+
+# if defined(__cplusplus)
 
 #define MRB_TRY(buf) try {
 #define MRB_CATCH(buf) } catch(mrb_jmpbuf_impl e) { if (e != (buf)->impl) { throw e; }
@@ -22,6 +24,10 @@
 #define MRB_THROW(buf) throw((buf)->impl)
 typedef mrb_int mrb_jmpbuf_impl;
 
+# else
+# error "need to be compiled with C++ compiler"
+# endif  /* __cplusplus */
+
 #else
 
 #include <setjmp.h>
@@ -46,9 +52,11 @@ typedef mrb_int mrb_jmpbuf_impl;
 struct mrb_jmpbuf {
   mrb_jmpbuf_impl impl;
 
-#if defined(MRB_ENABLE_CXX_EXCEPTION) && defined(__cplusplus)
+#if defined(MRB_ENABLE_CXX_EXCEPTION)
   static mrb_int jmpbuf_id;
+# if defined(__cplusplus)
   mrb_jmpbuf() : impl(jmpbuf_id++) {}
+# endif
 #endif
 };