Update make files so that the "-j" option will work.

Update make files so that the "-j" option will work.

On my machine (see trace below), with 4 cores, make is about 3 times quicker with the -j option.

=================

~/svn/stp-trunk$ make clean > /dev/null 2>&1
~/svn/stp-trunk$ time make > /dev/null 2>&1

real	0m45.615s
user	0m41.955s
sys	0m3.152s
~/svn/stp-trunk$ clean > /dev/null 2>&1
~/svn/stp-trunk$ time make -j > /dev/null 2>&1

real	0m17.616s
user	0m44.771s
sys	0m3.744s

git-svn-id: https://stp-fast-prover.svn.sourceforge.net/svnroot/stp-fast-prover/trunk/stp@351 e59a4935-1847-0410-ae03-e826735625c1