[Libreoffice] How can I speed up a build?

Hello all,

Two (probably idiotic) questions. First, when I refresh my master branch with

~/loroot $ ga pull -r

and then make via

~/loroot/rawbuild $ ./autogen.sh --with-num-cpus=5 --without-junit --disable-kde
~/loroot/rawbuild $ make

it seems that everything is rebuilt which takes hours, even if only a
few files have changed. Is there a way to ensure that only changed and
new files are compiled and linked, so the build process is a lot
quicker?

Second, although I'm using autogen.sh with the --with-num-cpus=5
option, it appears that only one cpu is being utilized. Or possibly
two at most - when I look at the output of "top" during the build, at
most I only ever see one "cc1plus" process and one "dmake" process,
and dmake is never using more that 1% of a cpu. Any thoughts?

Cheers,
Mattias