build: enable C++20

Elsewhere, all of our C++ repositories have updated to C++20.  Enable
it here as well.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I39ef53e7b41acd2f6b16070e3533006d6b4dadeb
diff --git a/configure.ac b/configure.ac
index a6b1453..2f8356b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -21,7 +21,7 @@
 )
 
 # Checks for typedefs, structures, and compiler characteristics.
-AX_CXX_COMPILE_STDCXX(17, [noext], [mandatory])
+AX_CXX_COMPILE_STDCXX(20, [noext], [mandatory])
 AX_APPEND_COMPILE_FLAGS([-fpic -Wall], [CFLAGS])
 AX_APPEND_COMPILE_FLAGS([-fpic -Wall], [CXXFLAGS])