build.sh: use CPU_COUNT rather than hardcoding 2 cpus
export TEMPO2=$PREFIX/share/tempo2
./configure --prefix=$PREFIX --disable-local --disable-psrhome PGPLOT_DIR=$PREFIX/include/pgplot
-make -j2
+make -j${CPU_COUNT}
make install
-make -j2 plugins
+make -j${CPU_COUNT} plugins
make plugins-install
# Copy runtime stuff
