glibc 2.30+ will actually include a definition for gettid() that makes
it so that users don't have to manually define a wrapper for it
themselves with syscall(). We don't currently check for this, and as a
result will end up redefining gettid() on the latest versions of glibc,
causing the build to fail:

FAILED: lib/76b5a35@@igt-igt_kmod_c@sta/igt_kmod.c.o
In file included from /usr/include/unistd.h:1170,
                 from ../../mnt/vol/lib/igt_core.h:43,
                 from ../../mnt/vol/lib/igt_kmod.c:28:
/usr/include/bits/unistd_ext.h:34:28: error: macro "gettid" passed 1 arguments, but takes just 0
   34 | extern __pid_t gettid (void) __THROW;
      |                            ^
In file included from ../../mnt/vol/lib/igt_kmod.c:27:
../../mnt/vol/lib/igt_aux.h:40: note: macro "gettid" defined here
   40 | #define gettid() syscall(__NR_gettid)
      |
[36/771] Compiling C object 'lib/76b5a35@@igt-igt_kms_c@sta/igt_kms.c.o'.
ninja: build stopped: subcommand failed.

So, fix this by by adding some meson checks to define HAVE_GETTID whenever the
host defines its own gettid(), and avoid redefining gettid() when HAVE_GETTID is
defined.

This fixes build igt-gpu-tools for me on Fedora Rawhide

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>