configure: Include a header in the check for _beginthread

Previously, the test tried compiling a call to the
_beginthread function without either declaring the function
(and its parameters) or including the corresponding header.

Since Clang 15 (which still is under development, so this may
still change before it's released) [1], implicit function
declarations are a hard error by default, when building code
in C99 mode (or newer).

[1] llvm/llvm-project@7d644e1

Author: mstorsjo

configure.ac

@@ -2846,7 +2846,7 @@ AC_MSG_RESULT([$with_nt_threads])
 if test $with_nt_threads = yes ; then
 AC_MSG_CHECKING([whether linking with nt-threads work])
 AC_LINK_IFELSE([
-    AC_LANG_PROGRAM([[]],[[_beginthread(0, 0, 0);]])
+    AC_LANG_PROGRAM([[#include <process.h>]],[[_beginthread(0, 0, 0);]])
   ],
   [AC_MSG_RESULT([yes])],
   [AC_MSG_ERROR([failed to link with nt-threads])])