Fix missing declarations of "POSIX" read()/write() on Windows with Clang

When (cross?) compiling this crate to Windows (in a Rust project) using
Clang 16 or newer, we're seeing these declaration errors:

    > cargo b --target x86_64-pc-windows-msvc
    warning: [email protected]: /usr/local/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/metis-sys-0.3.1/vendor/GKlib/io.c(63,18): error: call to undeclared function 'read'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    warning: [email protected]:    63 |     if ((rsize = read(fd, buf, tsize)) == -1)
    warning: [email protected]:       |                  ^
    ...
    warning: [email protected]: /usr/local/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/metis-sys-0.3.1/vendor/GKlib/io.c(84,17): error: call to undeclared function 'write'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    warning: [email protected]:    84 |     if ((size = write(fd, buf, tsize)) == -1)
    warning: [email protected]:       |                 ^

(LIHPC-Computational-Geometry/metis-rs#43 (comment))

Now it's yet unknown (we haven't checked) if older Clang had these
declarations in a header, or didn't treat this warning as error yet,
but the functions are POSIX which Windows isn't required to implement.
Fortunately they provide it but with a deprecation warning and a
recommended replacement of `_read()` (and `_write()`), but for this
`<io.h>` needs to be included:

https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/posix-read
https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/read

The same is true for `getpid()` that commit a8e7e25 ("port to vs2017")
used to provide a `win32/adapt.c/h` workaround for, but this can instead
be pulled from `<process.h>` (equally with a deprecation warning, but
we should probably address these all in a consistent manner instead
of defining a workaround).  Let's take this as a starting point and go
from there.

Author: MarijnS95

CMakeLists.txt

@@ -9,14 +9,7 @@ unset(abs)
 include(GKlibSystem.cmake)
 
 include_directories(".")
-if(MSVC)
-  include_directories("win32")
-  file(GLOB win32_sources RELATIVE "win32" "*.c")
-else(MSVC)
-  set(win32_sources, "")
-endif(MSVC)
-
-add_library(GKlib ${GKlib_sources} ${win32_sources})
+add_library(GKlib ${GKlib_sources})
 
 if(UNIX)
   target_link_libraries(GKlib m)

gk_arch.h

@@ -35,7 +35,9 @@
   #include "gk_ms_stdint.h"
   #include "gk_ms_inttypes.h"
   #include "gk_ms_stat.h"
-  #include "win32/adapt.h"
+  #include <io.h>
+  #include <process.h>
+  typedef int pid_t;
 #else
 #ifndef SUNOS
   #include <stdint.h>