build: Fix detecting pthread library.
6629a4b8015c
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Description

build: Fix detecting pthread library.

* configure.ac: Use pthread_cancel to detect pthread library.

In GNU C library, its routines support threading by changing its
behavior at runtime, based on whether or not an application links to
-lpthread. Some pthreads functions are in libc, others are in
libpthread.

Here, we try to detect the libpthread by testing a symbol which is
only available in libpthread (not libc). But, what symbol should be
tested may vary by GNU C library implementations.

So far, pthread_cancel is best one.