Add configure and build support for the pthreads library

Add configure and build support for the pthreads library

Support for pthreads can be forcibly disabled by passing
"--without-pthread" to the configure script.

The actual changes in this commit are in configure.ac and MCONFIG.in;
the other files were generated as a result of running aclocal,
autoconf, and autoheader on a Debian testing system.

Note: the autoconf-archive package must now be installed before
rerunning aclocal, to supply the AX_PTHREAD macro.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>