examples: Work around lack of mingw localtime_r()

mingw lacks localtime_r(); we were getting it from gnulib. But since
commit acf522e8 stopped linking examples against gnulib, we are
getting a build failure. Keep the examples standalone, and work
around mingw by using the non-reentrant localtime() (safe since our
examples are single-threaded), and add a necessary exemption to our
syntax check.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>