Add --with-shared-gc build flag

configure.ac

@@ -38,6 +38,7 @@ m4_include([tool/m4/ruby_replace_type.m4])dnl
 m4_include([tool/m4/ruby_require_funcs.m4])dnl
 m4_include([tool/m4/ruby_rm_recursive.m4])dnl
 m4_include([tool/m4/ruby_setjmp_type.m4])dnl
+m4_include([tool/m4/ruby_shared_gc.m4])dnl
 m4_include([tool/m4/ruby_stack_grow_direction.m4])dnl
 m4_include([tool/m4/ruby_thread.m4])dnl
 m4_include([tool/m4/ruby_try_cflags.m4])dnl
@@ -3750,6 +3751,7 @@ AS_IF([test x"$gcov" = xyes], [
 ])
 
 RUBY_SETJMP_TYPE
+RUBY_SHARED_GC
 }
 
 [begin]_group "installation section" && {
@@ -4656,6 +4658,7 @@ config_summary "target OS"           "$target_os"
 config_summary "compiler"            "$CC"
 config_summary "with thread"         "$THREAD_MODEL"
 config_summary "with coroutine"      "$coroutine_type"
+config_summary "with shared GC"      "$with_shared_gc"
 config_summary "enable shared libs"  "$ENABLE_SHARED"
 config_summary "dynamic library ext" "$DLEXT"
 config_summary "CFLAGS"              "$cflags"

tool/m4/ruby_shared_gc.m4

@@ -0,0 +1,19 @@
+dnl -*- Autoconf -*-
+AC_DEFUN([RUBY_SHARED_GC],[
+AC_ARG_WITH(shared-gc,
+    AS_HELP_STRING([--with-shared-gc],
+    [Enable replacement of Ruby's GC from a shared library.]),
+    [with_shared_gc=$withval], [unset with_shared_gc]
+)
+
+AC_SUBST([with_shared_gc])
+AC_MSG_CHECKING([if Ruby is build with shared GC support])
+AS_IF([test "$with_shared_gc" = "yes"], [
+    AC_MSG_RESULT([yes])
+    AC_DEFINE([USE_SHARED_GC], [1])
+], [
+    AC_MSG_RESULT([no])
+    with_shared_gc="no"
+    AC_DEFINE([USE_SHARED_GC], [0])
+])
+])dnl