#22 Fix C compatibility issues in the configure script

[Florian Weimer]

This is necessary to avoid incorrectly failing probes due to expected compiler errors.

Include <stdlib.h> for the exit function, and <string.h> for the strdup function.

Use fake prototypes to avoid implicit function declarations in the xine_* function probes.

[Sam James]

There's an issue with clock_gettime as well, I think?

conftest.c:62:9: warning: "_GNU_SOURCE" redefined
   62 | #define _GNU_SOURCE /**/
      |         ^~~~~~~~~~~
conftest.c:23:9: note: this is the location of the previous definition
   23 | #define _GNU_SOURCE 1
      |         ^~~~~~~~~~~
conftest.c: In function 'main':
conftest.c:83:10: error: implicit declaration of function 'clock_gettime' [-Wimplicit-function-declaration]
   83 |          clock_gettime(CLOCK_REALTIME, &ts);
      |          ^~~~~~~~~~~~~
conftest.c:83:10: warning: nested extern declaration of 'clock_gettime' [-Wnested-externs]
conftest.c:83:24: error: 'CLOCK_REALTIME' undeclared (first use in this function)
   83 |          clock_gettime(CLOCK_REALTIME, &ts);
      |                        ^~~~~~~~~~~~~~

Need #include <time.h>.

[Florian Weimer]

Good catch, but that's already not working with any compiler because CLOCK_REALTIME is undefined. We are focusing mostly on regressions, especially with the current instrumentation that suppresses error logging in case of unrelated errors.