c-stack: output diagnostic in single 'write'

* lib/c-stack.c (die): In the typical case, use just one 'write'
syscall to output the diagnostic, as this lessens interleaving.
(die, c_stack_action): Assume C99.
* modules/c-stack (Depends-on): Add c99, mempcpy.

modules/c-stack

@@ -7,6 +7,7 @@ lib/c-stack.c
 m4/c-stack.m4
 
 Depends-on:
+c99
 errno
 exitfail
 getprogname
@@ -15,6 +16,7 @@ ignore-value
 intprops
 inttypes
 libsigsegv
+mempcpy
 raise
 sigaction
 stdbool