libc: Fix buglet around __progname
508b20ead1710e8c295630d096960e51a65b7e5d disentangled
program_invocation_name and __progname but forgot to make __progname
conditional on __UCLIBC_HAS___PROGNAME__, resulting in getconf
attempting to re-define __progname in case __UCLIBC_HAS___PROGNAME__ was
not defined.
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>