configure: Use headers in the check for _beginthreadex for w32threads

When targeting the metro API subset, this function still exists in
the link libraries, but is excluded from the headers. This makes
sure w32threads is automatically disabled when targeting this API
subset (since not all the necessary functions for it are available).

Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -3332,7 +3332,7 @@ disabled  zlib || check_lib   zlib.h      zlibVersion -lz   || disable  zlib
 disabled bzlib || check_lib2 bzlib.h BZ2_bzlibVersion -lbz2 || disable bzlib
 
 if ! disabled w32threads && ! enabled pthreads; then
-    check_func _beginthreadex && enable w32threads
+    check_func_headers "windows.h process.h" _beginthreadex && enable w32threads
 fi
 
 # check for some common methods of building with pthread support