Excuse me, looking at the logs I see that the function names are still in quotes.
% grep -E ' implicit|call to undeclared' config.log.*
config.log.clang15:conftest.c:62:6: warning: call to undeclared library function 'exit' with type 'void (int) __attribute__((noreturn))'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
config.log.clang15:conftest.c:150:5: warning: call to undeclared library function 'exit' with type 'void (int) __attribute__((noreturn))'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
config.log.clang15:conftest.c:144:7: warning: call to undeclared library function 'strcmp' with type 'int (const char *, const char *)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
config.log.clang15:conftest.c:145:5: warning: call to undeclared library function 'exit' with type 'void (int) __attribute__((noreturn))'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
config.log.clang15:conftest.c:167:5: warning: call to undeclared library function 'exit' with type 'void (int) __attribute__((noreturn))'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
config.log.clang15:conftest.c:174:23: warning: call to undeclared function 'pthread_condattr_setclock'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
config.log.clang15:conftest.c:174:1: warning: call to undeclared function 'pthread_cond_timedwait_monotonic'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
config.log.clang15:conftest.c:174:1: warning: call to undeclared function 'pthread_cond_timedwait_monotonic_np'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
config.log.xcode13:conftest.c:62:6: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
config.log.xcode13:conftest.c:62:6: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
config.log.xcode13:conftest.c:62:6: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
config.log.xcode13:conftest.c:149:5: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
config.log.xcode13:conftest.c:142:7: error: implicitly declaring library function 'strcmp' with type 'int (const char *, const char *)' [-Werror,-Wimplicit-function-declaration]
config.log.xcode13:conftest.c:143:5: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
config.log.xcode13:conftest.c:164:5: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
config.log.xcode13:conftest.c:170:23: error: implicit declaration of function 'pthread_condattr_setclock' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
config.log.xcode13:conftest.c:170:1: error: implicit declaration of function 'pthread_cond_timedwait_monotonic' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
config.log.xcode13:conftest.c:170:1: error: implicit declaration of function 'pthread_cond_timedwait_monotonic_np' is invalid in C99 [-Werror,-Wimplicit-function-declaration]