Drop checks for pid_t from autoconf and CMake

This type should always be defined in sys/types.h on any non-ancient
Unix system, so don't bother checking for it, which is not only wasteful
but can even be harmful because it can conflict with pid_t definitions
in the other libraries under Windows, where we (wrongly) define it as
int (which is actually a bug in autoconf, which was recently fixed, see
https://savannah.gnu.org/support/index.php?110296) when using CMake.

So just don't define it at all: it should be already defined under Unix
and we don't use it under MSW anyhow.

See https://github.com/microsoft/vcpkg/issues/19110

See #18150.

setup.h_vms

@@ -45,9 +45,6 @@
 /* Define to `long' if <sys/types.h> doesn't define.  */
 #undef off_t
 
-/* Define to `int' if <sys/types.h> doesn't define.  */
-#undef pid_t
-
 /* Define to `unsigned' if <sys/types.h> doesn't define.  */
 #undef size_t