Fix syscall() undeclared error if built with stricter build options

[lumag]

If cpuinfo is built with a stricer build flags, it fails on some systems with the undeclared function error, because on Glibc syscall() is not a part of the headers unless one of the feature macros is defined. Add the least intrusive one, _DEFAULT_SOURCE.

cpuinfo/src/api.c:319:23: error: call to undeclared function 'syscall'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
if CPUINFO_UNLIKELY(syscall(__NR_getcpu, &cpu, NULL, NULL) != 0) {
^
cpuinfo/src/api.c:338:23: error: call to undeclared function 'syscall'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
if CPUINFO_UNLIKELY(syscall(__NR_getcpu, &cpu, NULL, NULL) != 0) {

Closes #171

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[fbarchard]

seems okay but is there a way to reproduce the 'strict build flags'?

[lumag]

I stumbled upon the issue when building TensorFlow-lite for ARMv7 platforms in Yocto.

[fbarchard]

i asked gemini and it says this is the better fix:

#define _GNU_SOURCE // This is the "gold standard" for Linux/glibc systems
#include <unistd.h>
#include <sys/syscall.h>

[lumag]

i asked gemini and it says this is the better fix:

#define _GNU_SOURCE // This is the "gold standard" for Linux/glibc systems #include <unistd.h> #include <sys/syscall.h>

_GNU_SOURCE enables GNU Glibc extensions. _DEFAULT_SOURCE doesn't enable those and it's enough for this case.

manpage also recommends just _DEFAULT_SOURCE.
Net result: don't use Gemini for programming, it might hallucinate.