Suppress C++20-compat warnings

The warnings (errors due to -Werror) are coming from CBMC. The issue has
been introduced in gcc 13 and is now fixed on CBMC develop branch in
commit 3a6cef0 which can't be cleanly applied to the version that 2LS is
currently using. With no current plans to migrate to C++-20, we can
ignore the warning for now.

Author: FrNecas

src/config.inc

@@ -3,7 +3,7 @@ CPROVER_DIR = ../../lib/cbmc
 # Variables you may want to override
 
 # Enable warnings
-CXXFLAGS += -Wall -Werror -Wno-long-long -Wno-sign-compare -Wno-parentheses -Wno-strict-aliasing -pedantic
+CXXFLAGS += -Wall -Werror -Wno-long-long -Wno-sign-compare -Wno-parentheses -Wno-c++20-compat -Wno-strict-aliasing -pedantic
 
 # Select optimisation or debug
 #CXXFLAGS += -O2