Makefile: -lpthread may still be necessary when libc has only pthread stubs

Makefile: -lpthread may still be necessary when libc has only pthread stubs

Without this patch, systems that provide stubs for pthread functions
in libc, but which still require libpthread for full the pthread
implementation are not detected correctly.

Also, some systems require -pthread in CFLAGS for each compilation
unit for a successful link of an mt binary, which is also addressed by
this patch.

Signed-off-by: Gary V. Vaughan <gary@thewrittenword.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>