popt: fix build issue due to glob functions on Blackfin

While the Blackfin external uClibc toolchains have <glob.h>, they
don't implement all the functions needed by popt, causing build
failures. One solution would have been to disable popt with such
toolchains, but this would have meant propagating this additional
dependency to the numerous reverse dependencies of popt. Instead, this
commit chooses to make popt build on Blackfin external toolchains by
disabling the usage of <glob.h> functions altogether.

Fixes:

  http://autobuild.buildroot.org/results/bfc/bfcb2ed27debafc584e133f5ae11ad2061ad2b16/build-end.log
  http://autobuild.buildroot.org/results/410/410c8f3a0c7ca2c7098a47c30088038411635ae4/build-end.log



and gazillion of similar failures.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>