While compiling a very recent release of Python (ie. 3.9.0rc2) with a fairly old release of gcc (ie. debian 6.0.6 gcc 4.4.5) I get the following error :

gcc -pthread -c -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall    -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal  -I. -I./Include    -DPy_BUILD_CORE -o Parser/pegen/pegen.o Parser/pegen/pegen.c

Parser/pegen/pegen.c: In function ‘_PyPegen_seq_count_dots’:
Parser/pegen/pegen.c:1414: error: implicit declaration of function ‘__builtin_unreachable’

Indeed, this '__builtin_unreachable()' function will exists starting from gcc 4.5.

The configure file could detect this and offer an alternative.

(does a table of the supported gcc release for each python versions exists ?)