Fix link error on Linux/musl.

get_nprocs() is a GNU convenience function equivalent to POSIX2008
sysconf(_SC_NPROCESSORS_ONLN); the latter should be available in unistd.h
on any current *nix. (OS X supports this call since 10.5, and FreeBSD
currently supports it. But this commit does not change FreeBSD or OS X
versions.)

driver/others/memory.c

@@ -162,7 +162,7 @@ int get_num_procs(void);
 #else
 int get_num_procs(void) {
   static int nums = 0;
-  if (!nums) nums = get_nprocs();
+  if (!nums) nums = sysconf(_SC_NPROCESSORS_ONLN);
   return nums;
 }
 #endif