configure: rework pthread handling to allow for user defined flags

configure: rework pthread handling to allow for user defined flags

The tests for POSIX threads can now be controlled by the user with the
--enable-pthreads=FLAGS option.  If this is set (to some value other
than yes or no), the value is passed to the compiler.  Thread support
is based solely on the outcome of this test.  The user may specify not
to use threading at all or to use the default tests (first -pthread
then -lpthread) by not specifying FLAGS when passing --enable-pthreads.

Signed-off-by: Ben Walton <bwalton@artsci.utoronto.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>