configure.ac: fix the check for static libcap

configure.ac: fix the check for static libcap

The existing check doesn't work, because when you statically
link a program against libc, any functions not called are not
included.  So cap_init() which we check for is not there in
the built binary.

So instead just check whether a "gcc -lcap -static" works.
If libcap.a is not available it will fail, if it is it will
succeed.

Signed-off-by: Serge Hallyn <shallyn@cisco.com>