add support for program_invocation[_short]_name

this is a bit ugly, and the motivation for supporting it is
questionable. however the main factors were:
1. it will be useful to have this for certain internal purposes
anyway -- things like syslog.
2. applications can just save argv[0] in main, but it's hard to fix
non-portable library code that's depending on being able to get the
invocation name without the main application's help.

include/errno.h

@@ -5,6 +5,8 @@
 extern "C" {
 #endif
 
+#include <features.h>
+
 #include <bits/errno.h>
 
 #ifdef __GNUC__
@@ -13,6 +15,10 @@ __attribute__((const))
 int *__errno_location(void);
 #define errno (*__errno_location())
 
+#ifdef _GNU_SOURCE
+extern char *program_invocation_short_name, *program_invocation_name;
+#endif
+
 #ifdef __cplusplus
 }
 #endif