On a system using an older version of gcc (v5.7.4) I get an error: (also AIX 6.1)

        gcc -pthread -Wno-unused-result -Wsign-compare -g -O0 -Wall  -O  -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration  -I./Include/internal  -I. -I./Include    -DPy_BUILD_CORE_BUILTIN  -DPy_BUILD_CORE -I./Include/internal -c ./Modules/posixmodule.c -o Modules/posixmodule.o
./Modules/posixmodule.c: In function 'os_preadv_impl':
./Modules/posixmodule.c:8765:9: error: implicit declaration of function 'preadv' [-Werror=implicit-function-declaration]
./Modules/posixmodule.c: In function 'os_pwritev_impl':
./Modules/posixmodule.c:9336:9: error: implicit declaration of function 'pwritev' [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors

On another system - same code base (commit e7a4bb554edb72fc6619d23241d59162d06f249a) no "error" status. (AIX 7.2)

Not knowing gcc I have no idea which is correct - maybe they both are because the second system (where I am a guest) has an explicit declaration of these routines.

Is there a flag I can add to gcc to get the "in-line" pre-processing output so I can look for differences with regard to these two functions?

Thanks for hint.