fix: disable stringop-overread for gcc11

This workaround let enable -Werror -Wall on our CI, otherwise compilation of uint256.h fails with this error:

    In member function ‘constexpr int base_blob<BITS>::Compare(const base_blob<BITS>&) const [with unsigned int BITS = 256]’,
        inlined from ‘constexpr bool operator!=(const base_blob<256>&, const base_blob<256>&)’ at ./uint256.h:58:96,
        inlined from ‘bool CDeterministicMNManager::BuildNewListFromBlock(const CBlock&, gsl::not_null<const CBlockIndex*>, BlockValidationState&, const CCoinsViewCache&, CDeterministicMNList&, llmq::CQuorumSnapshotManager&, bool)’ at evo/deterministicmns.cpp:818:119:
    ./uint256.h:55:77: error: ‘int __builtin_memcmp_eq(const void*, const void*, long unsigned int)’ specified bound 32 exceeds source size 0 [-Werror=stringop-overread]
       55 |     constexpr int Compare(const base_blob& other) const { return std::memcmp(m_data.data(), other.m_data.data(), WIDTH); }
          |                                                                  ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    cc1plus: all warnings being treated as errors

Author: knst

configure.ac

@@ -457,6 +457,15 @@ if test "x$enable_werror" = "xyes"; then
   AX_CHECK_COMPILE_FLAG([-Werror=return-type], [], [ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Wno-error=return-type"], [$CXXFLAG_WERROR],
                         [AC_LANG_SOURCE([[#include <cassert>
                                           int f(){ assert(false); }]])])
+  dnl -Wstringop-overread is broken in GCC 11.
+  AC_COMPILE_IFELSE(
+    [AC_LANG_PROGRAM([[]],
+         [[#if __GNUC__ == 11
+           #error -Wstringop-overread is broken in GCC 11
+           #endif
+          ]])],
+    [],
+    [ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Wno-stringop-overread"])
 fi
 
 if test "x$CXXFLAGS_overridden" = "xno"; then