diff --git a/ruby-3.1.0-autoconf-2.70-add-ac-prog-cc.patch b/ruby-3.1.0-autoconf-2.70-add-ac-prog-cc.patch
new file mode 100644
index 0000000..1bd20bd
--- /dev/null
+++ b/ruby-3.1.0-autoconf-2.70-add-ac-prog-cc.patch
@@ -0,0 +1,27 @@
+From 912a8dcfc5369d840dcd6bf0f88ee0bac7d902d6 Mon Sep 17 00:00:00 2001
+From: Nobuyoshi Nakada <nobu@ruby-lang.org>
+Date: Thu, 30 Sep 2021 18:24:37 +0900
+Subject: [PATCH] Needs `AC_PROG_CC`
+
+Although `AC_PROG_CC_C99` has been obsolete, `AC_PROG_CC` is not
+and the latter is necessary not to make C++ compiler mandatory.
+---
+ configure.ac | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/configure.ac b/configure.ac
+index b24a8f59b0..c7059ee1ec 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -203,7 +203,7 @@ rb_test_CXXFLAGS=${CXXFLAGS+yes}
+ # BSD's ports and MacPorts prefix GNU binutils with 'g'
+ 
+ dnl Seems necessarily in order to add -std=gnu99 option for gcc 4.9.
+-m4_version_prereq([2.70], [], [AC_PROG_CC_C99])
++m4_version_prereq([2.70], [AC_PROG_CC], [AC_PROG_CC_C99])
+ 
+ AC_PROG_CXX
+ AC_PROG_CPP
+-- 
+2.31.1
+
diff --git a/ruby.spec b/ruby.spec
index 414eb19..eb43d42 100644
--- a/ruby.spec
+++ b/ruby.spec
@@ -164,6 +164,9 @@ Patch18: ruby-3.1.0-addr2line-DW_FORM_ref_addr.patch
 # Avoid possible timeout errors in TestBugReporter#test_bug_reporter_add.
 # https://bugs.ruby-lang.org/issues/16492
 Patch19: ruby-2.7.1-Timeout-the-test_bug_reporter_add-witout-raising-err.patch
+# Add AC_PROG_CC to make C++ compiler dependency optional on autoconf >= 2.70.
+# https://github.com/ruby/ruby/commit/912a8dcfc5369d840dcd6bf0f88ee0bac7d902d6
+Patch20: ruby-3.1.0-autoconf-2.70-add-ac-prog-cc.patch
 
 Requires: %{name}-libs%{?_isa} = %{version}-%{release}
 Suggests: rubypick
@@ -614,6 +617,7 @@ rm -rf ext/fiddle/libffi*
 %patch17 -p1
 %patch18 -p1
 %patch19 -p1
+%patch20 -p1
 
 # Provide an example of usage of the tapset:
 cp -a %{SOURCE3} .