diff --git a/common.mk b/common.mk
index 1e863a6627..bf60ede930 100644
--- a/common.mk
+++ b/common.mk
@@ -43,7 +43,7 @@ RUN_OPTS      = --disable-gems
 
 # GITPULLOPTIONS = --no-tags
 
-INCFLAGS = -I. -I$(arch_hdrdir) -I$(hdrdir) -I$(srcdir) -I$(UNICODE_HDR_DIR)
+INCFLAGS = -I. -I$(arch_hdrdir) -I$(hdrdir) -I$(srcdir) -I$(UNICODE_HDR_DIR) $(incflags)
 
 GEM_HOME =
 GEM_PATH =
diff --git a/configure.ac b/configure.ac
index 747c25932e..b687101967 100644
--- a/configure.ac
+++ b/configure.ac
@@ -978,6 +978,7 @@ AS_IF([test "x$OPT_DIR" != x], [
     INCFLAGS="${INCFLAGS:+$INCFLAGS }"`echo "$OPT_DIR" | tr "${PATH_SEPARATOR}" '\012' |
         sed '/^$/d;s|^|-I|;s|$|/include|' | tr '\012' ' ' | sed 's/ *$//'`
 ])
+AC_SUBST(incflags, "$INCFLAGS")
 
 test -z "${ac_env_CFLAGS_set}" -a -n "${cflags+set}" && eval CFLAGS="\"$cflags $ARCH_FLAG\""
 test -z "${ac_env_CXXFLAGS_set}" -a -n "${cxxflags+set}" && eval CXXFLAGS="\"$cxxflags $ARCH_FLAG\""
diff --git a/template/Makefile.in b/template/Makefile.in
index 1dff601d49..e189d88c3f 100644
--- a/template/Makefile.in
+++ b/template/Makefile.in
@@ -89,6 +89,7 @@ optflags = @optflags@
 debugflags = @debugflags@
 warnflags = @warnflags@ @strict_warnflags@
 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)
diff --git a/version.h b/version.h
index 04c418fc01..f8d748218e 100644
--- a/version.h
+++ b/version.h
@@ -11,7 +11,7 @@
 # define RUBY_VERSION_MINOR RUBY_API_VERSION_MINOR
 #define RUBY_VERSION_TEENY 5
 #define RUBY_RELEASE_DATE RUBY_RELEASE_YEAR_STR"-"RUBY_RELEASE_MONTH_STR"-"RUBY_RELEASE_DAY_STR
-#define RUBY_PATCHLEVEL 230
+#define RUBY_PATCHLEVEL 231
 
 #include "ruby/version.h"
 #include "ruby/internal/abi.h"