Set proctitle: avoid the use of __attribute__((constructor)).

This cased a segfault in some Linux system and was GCC-specific.

Commit modified by @antirez:

1) Stripped away the part to set the proc title via config for now.
2) Handle initialization of setproctitle only when the replacement
   is used.
3) Don't require GCC now that the attribute constructor is no
   longer used.

src/config.h

@@ -114,6 +114,8 @@
 
 #if (defined __linux || defined __APPLE__)
 #define USE_SETPROCTITLE
+#define INIT_SETPROCTITLE_REPLACEMENT
+void spt_init(int argc, char *argv[]);
 void setproctitle(const char *fmt, ...);
 #endif