Build: Enable Automake's parallel test harness.

It has been the default for quite some time already and
the old serial harness isn't discouraged. The downside is
that with parallel tests one cannot print progress info or
other diagnostics to the terminal; all output from the tests
will be in the log files only. But now that the compression
tests are separated the parallel tests will speed things up.

configure.ac

@@ -570,7 +570,7 @@ echo "Initializing Automake:"
 # http://debbugs.gnu.org/cgi/bugreport.cgi?bug=17354
 # The -Wno-unsupported is used to silence warnings about missing
 # "subdir-objects".
-AM_INIT_AUTOMAKE([1.12 foreign tar-v7 filename-length-max=99 serial-tests -Wno-unsupported])
+AM_INIT_AUTOMAKE([1.12 foreign tar-v7 filename-length-max=99 -Wno-unsupported])
 AC_PROG_LN_S
 
 AC_PROG_CC_C99