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.

build/cmake/setup.cmake

@@ -648,7 +648,7 @@ endif()
 
 # Check size and availability of various types
 set(SYSTYPES
-    pid_t size_t
+    size_t
     wchar_t int long short
     gid_t uid_t
     )