--- /dev/null
+From 3f6fc35cc28387e327cb6f47feb00061ed17b84c
+From: Mamoru TASAKA <mtasaka@fedoraproject.org>
+Date: Sat, 30 Apr 2022 13:32:34 +0900
+Subject: [PATCH] configure.ac: reset compiler flags before calling
+ AC_CHECK_DECLS
+
+With autoconf 2.71, [AC_CHECK_DECLS] calls new internal macro
+[_AC_UNDECLARED_BUILTIN], which raises error with unknown compiler
+flag. On non-x86 archs, -mavx or so is not recognized, so configure
+fails on such architecture.
+
+To fix this, reset vector related compiler flags before calling
+AC_CHECK_DECLS.
+
+*OPENWRT EDIT: typo "sse_flags"
+---
+ configure.ac | 16 ++++++++++++----
+ 1 file changed, 12 insertions(+), 4 deletions(-)
+
+--- a/configure.ac
++++ b/configure.ac
+@@ -674,11 +674,19 @@ AC_CHECK_HEADERS([sys/types.h sys/un.h a
+
+ save_cxxflags="${CXXFLAGS}"
+ save_cppflags="${CPPFLAGS}"
+-CXXFLAGS="${CXXFLAGS} -msse3"
+-CPPFLAGS="${CPPFLAGS} -msse3"
++sse3_flags="-msse3"
++avx_flags="-mavx"
++CXXFLAGS="${save_cxxflags} ${sse3_flags}"
++CPPFLAGS="${save_cppflags} ${sse3_flags}"
++AC_LINK_IFELSE([AC_LANG_PROGRAM([],)], [], [sse3_flags=""])
++CXXFLAGS="${save_cxxflags} ${avx_flags}"
++CPPFLAGS="${save_cppflags} ${avx_flags}"
++AC_LINK_IFELSE([AC_LANG_PROGRAM([],)], [], [avx_flags=""])
++CXXFLAGS="${save_cxxflags} ${sse3_flags}"
++CXXFLAGS="${save_cxxflags} ${sse3_flags}"
+ AC_CHECK_HEADERS([intrin.h x86intrin.h pmmintrin.h xmmintrin.h emmintrin.h])
+-CXXFLAGS="${save_cxxflags} -mavx"
+-CPPFLAGS="${save_cppflags} -mavx"
++CXXFLAGS="${save_cxxflags} ${avx_flags}"
++CPPFLAGS="${save_cppflags} ${avx_flags}"
+ AC_CHECK_HEADERS([immintrin.h avxintrin.h])
+
+ AC_CHECK_DECLS([_xgetbv, xgetbv, __xgetbv, cpuid, _cpuid, __cpuid],