Fix clang build for Ruby 2.6

While RUBY_METHOD_FUNC causes problems with newer Rubies, it's still
needed under Ruby 2.6 with clang.  We'll define our own macro to
make it a no-op under Ruby 2.7+.

Tested with clang 10.0.0-4ubuntu1, 11.0.0-2~ubuntu20.04.1, and
12.0.0-3ubuntu1~20.04.4 on Ubuntu 20.04.3 LTS (x86-64).

Fixes: 7a16ef8 ("remove RUBY_METHOD_FUNC use")
Fixes: #223

ext/mini_racer_extension/extconf.rb

@@ -75,4 +75,9 @@
   $LDFLAGS.insert(0, " -fsanitize=address ")
 end
 
+# there doesn't seem to be a CPP macro for this in Ruby 2.6:
+if RUBY_ENGINE == 'ruby'
+  $CPPFLAGS += ' -DENGINE_IS_CRUBY '
+end
+
 create_makefile 'mini_racer_extension'

ext/mini_racer_extension/mini_racer_extension.cc

@@ -2,6 +2,7 @@
 #include <ruby.h>
 #include <ruby/thread.h>
 #include <ruby/io.h>
+#include <ruby/version.h>
 #include <v8.h>
 #include <v8-profiler.h>
 #include <libplatform/libplatform.h>
@@ -13,6 +14,14 @@
 #include <math.h>
 #include <errno.h>
 
+/* workaround C Ruby <= 2.x problems w/ clang in C++ mode */
+#if defined(ENGINE_IS_CRUBY) && \
+	RUBY_API_VERSION_MAJOR == 2 && RUBY_API_VERSION_MINOR <= 6
+#  define MR_METHOD_FUNC(fn) RUBY_METHOD_FUNC(fn)
+#else
+#  define MR_METHOD_FUNC(fn) fn
+#endif
+
 using namespace v8;
 
 typedef struct {
@@ -1285,8 +1294,8 @@ gvl_ruby_callback(void* data) {
     VALUE callback_data_value = (VALUE)&callback_data;
 
     // TODO: use rb_vrescue2 in Ruby 2.7 and above
-    result = rb_rescue2(protected_callback, callback_data_value,
-            rescue_callback, callback_data_value, rb_eException, (VALUE)0);
+    result = rb_rescue2(MR_METHOD_FUNC(protected_callback), callback_data_value,
+            MR_METHOD_FUNC(rescue_callback), callback_data_value, rb_eException, (VALUE)0);
 
     if(callback_data.failed) {
         rb_iv_set(parent, "@current_exception", result);