build: enable C++20

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I34badb788f3d1aeba4ef256a5f811cd92148f0ed
diff --git a/configure.ac b/configure.ac
index 12762f0..fc78994 100644
--- a/configure.ac
+++ b/configure.ac
@@ -35,7 +35,7 @@
 PKG_INSTALLDIR
 
 # 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([-Wall -Wextra -Wpedantic], [CFLAGS])
 AX_APPEND_COMPILE_FLAGS([-Wall -Wextra -Wpedantic], [CXXFLAGS])