diff --git a/gc.c b/gc.c
index 19e65a1f14..67857ed44c 100644
--- a/gc.c
+++ b/gc.c
@@ -707,6 +707,7 @@ typedef struct gc_function_map {
 static rb_gc_function_map_t rb_gc_functions;
 
 # define RUBY_GC_LIBRARY "RUBY_GC_LIBRARY"
+# define MODULAR_GC_DIR STRINGIZE(modular_gc_dir)
 
 static void
 ruby_modular_gc_init(void)
diff --git a/template/Makefile.in b/template/Makefile.in
index 39d5bdd156..25c788f565 100644
--- a/template/Makefile.in
+++ b/template/Makefile.in
@@ -97,7 +97,7 @@ cppflags = @cppflags@
 incflags = @incflags@
 RUBY_DEVEL = @RUBY_DEVEL@ # "yes" or empty
 _RUBY_DEVEL_enabled = $(RUBY_DEVEL:no=)
-XCFLAGS = @XCFLAGS@ $(INCFLAGS) $(_RUBY_DEVEL_enabled:yes=-DRUBY_DEVEL=1)
+XCFLAGS = @XCFLAGS@ $(INCFLAGS) $(_RUBY_DEVEL_enabled:yes=-DRUBY_DEVEL=1) -Dmodular_gc_dir="$(modular_gc_dir)"
 USE_RUBYGEMS = @USE_RUBYGEMS@
 USE_RUBYGEMS_ = $(USE_RUBYGEMS:yes=)
 CPPFLAGS = @CPPFLAGS@ $(USE_RUBYGEMS_:no=-DDISABLE_RUBYGEMS=1)
diff --git a/win32/Makefile.sub b/win32/Makefile.sub
index 31a5413017..09b8debd76 100644
--- a/win32/Makefile.sub
+++ b/win32/Makefile.sub
@@ -156,6 +156,9 @@ DEBUGFLAGS = -Zi
 !if "$(RUBY_DEVEL)" == "yes"
 XCFLAGS = $(XCFLAGS) -DRUBY_DEVEL=1
 !endif
+!if "$(modular_gc_dir)" != ""
+XCFLAGS = $(XCFLAGS) -Dmodular_gc_dir="$(modular_gc_dir)"
+!endif
 !if !defined(OPTFLAGS)
 !if $(MSC_VER) < 1400
 OPTFLAGS = -O2b2xg-