configure: allow checking multiple functions in check_func_headers()

This makes it possible to pass a space-separated list of functions
to check_func_headers and check_lib2.  If any function is missing,
none are enabled as available, so this should only be used for
all-or-nothing sets, i.e. groups in which none will be used if any
one is missing.

Signed-off-by: Mans Rullgard <mans@mansr.com>