progname: also set global program_invocation_name, when possible.

progname: also set global program_invocation_name, when possible.

Before this change, a libtool-enabled program that calls glibc's
error function would report the program name as
"/abs/dir/.libs/lt-program_name" rather than the desired program_name.
* modules/progname (configure.ac): Check for a declaration of
program_invocation_name.
* lib/progname.c:  Include <errno.h>.
(set_program_name) [HAVE_DECL_PROGRAM_INVOCATION_NAME]:
Set program_invocation_name.