How to fix error: ‘nullptr’ was not declared in this scope - c++

Question

I am trying to build gcc-12.2.0 using documentation given in lfs (linux from scratch) https://www.linuxfromscratch.org/blfs/view/svn/general/gcc.html. I am coming across the error: ‘nullptr’ was not declared in this scope. I am running gcc-7.5.0 on ubuntu-18.04.
A bit of background to the error is that there first a warning
../../../gcc-12.2.0/libcpp/include/cpplib.h:1141:1: warning: scoped enums only available with -std=c++11 or -std=gnu++11
enum class CPP_time_kind.
Then the errror (4 times)
../../../gcc-12.2.0/libcpp/files.cc: In function ‘bool _cpp_stack_file(cpp_reader*, _cpp_file*, include_type, location_t)’:
../../../gcc-12.2.0/libcpp/files.cc:910:15: error: ‘nullptr’ was not declared in this scope
char *buf = nullptr;
../../../gcc-12.2.0/libcpp/files.cc: In function ‘_cpp_file* test_header_unit(cpp_reader*, const char*, bool, location_t)’:
../../../gcc-12.2.0/libcpp/files.cc:1125:10: error: ‘nullptr’ was not declared in this scope
return nullptr;
../../../gcc-12.2.0/libcpp/files.cc: In function ‘const char* _cpp_find_header_unit(cpp_reader*, const char*, bool, location_t)’:
../../../gcc-12.2.0/libcpp/files.cc:1150:10: error: ‘nullptr’ was not declared in this scope
return nullptr;
../../../gcc-12.2.0/libcpp/files.cc: In function ‘const char* cpp_probe_header_unit(cpp_reader*, const char*, bool, location_t)’:
../../../gcc-12.2.0/libcpp/files.cc:1163:10: error: ‘nullptr’ was not declared in this scope
return nullptr;
All the posts I have googled relate to this error appearing in the running of a specific program using c++ or g++, not in compiling gcc.
I have no idea how to include -std=c++11 in my gcc build.
Assistance in solving this problem will be appreciated.