configure: Only test the makecontext signature we need

The test/explicit-bzero.c test uses a start routine without any
arguments.  There is no need for the multi-argument version.

This avoids a build failure with glibc and future compilers.
The GNU C library declares the makecontext callback of
type void (*) (void), so no cast is needed.  On other systems,
the type may be the (currently distinct) type void (*) (),
but given that this only affects the ability to execute a test,
no further machinery is added here to detect that different type.

configure.ac

@@ -303,13 +303,11 @@ AS_IF([test $ac_cv_header_ucontext_h = yes],
 #include <ucontext.h>
 static int x;
 static void fn1(void) {}
-static void fn2(int a, int b) { x = a - b; }
 ]], [[
   ucontext_t uc1, uc2;
   if (getcontext(&uc1)) return 1;
   if (setcontext(&uc1)) return 1;
   makecontext(&uc1, fn1, 0);
-  makecontext(&uc2, fn2, 2, 1, 1);
   if (swapcontext(&uc1, &uc2)) return 1;
   return x;
 ]])],