autoconf-archive: manually patch C++20 support

autoconf-archive added support for C++20 finally in v2022.02.11
but none of the Ubuntu distro versions have picked this up
yet (checked impish and jammy).  Manually patch the file so that
we can upgrade our autoconf-based C++ repositories to use C++20.

The v2022.02.11 version is already included in Yocto.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ied06de39741379689db153896a9a497cb6df229c
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index 2a61834..1a744e8 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -833,6 +833,10 @@
     rsync \
     libcryptsetup-dev
 
+# Apply autoconf-archive-v2022.02.11 file ax_cxx_compile_stdcxx for C++20.
+RUN curl "http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_cxx_compile_stdcxx.m4;hb=3311b6bdeff883c6a13952594a9dcb60bce6ba80" \
+  > /usr/share/aclocal/ax_cxx_compile_stdcxx.m4
+
 RUN npm install -g eslint@latest eslint-plugin-json@latest
 
 RUN update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-11 11 \