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.

configure

@@ -29757,17 +29757,6 @@ _ACEOF
 
 fi
 
-ac_fn_c_check_type "$LINENO" "pid_t" "ac_cv_type_pid_t" "$ac_includes_default"
-if test "x$ac_cv_type_pid_t" = xyes; then :
-
-else
-
-cat >>confdefs.h <<_ACEOF
-#define pid_t int
-_ACEOF
-
-fi
-
 ac_fn_c_check_type "$LINENO" "size_t" "ac_cv_type_size_t" "$ac_includes_default"
 if test "x$ac_cv_type_size_t" = xyes; then :