I recently installed LibreOffice on my Slackware64-current system without downloading the dependencies externally (I had most of them installed for other programs and I saw no point in externally fetching them or the other like...2 dependencies I didn't have so I installed them directly onto the system).
Other than that, the only dependencies I can think of that's not a part of a default Slackware installation was:
liborcus (use 0.3.0), libwps, libwpg, libvisio, mythes, hyphen, lp_solve, cppunit, graphite2, libexttextcat, liblangtag, libmspub, mdds, perl-Archive-Zip, vigra, lxml and ucpp (these are dependencies for my configuration options below).
Off the top of my head, vigra, lxml, cppunit and perl-Archive-Zip are on SlackBuilds.org (maybe a few others are too).
The rest you have to build yourself (most are just basic ./configure ; make ; make install programs except things like lp_solve, you can find more information in their documentation).
Here are the configure options I used (except I used ./autogen.sh --some-option instead of ./configure --some-option) to help you out:
--with-system-libs --with-system-ucpp --with-x --with-external-hyph-dir=/opt/share/hyphen --with-lang="$LANGUAGES"
--with-parallelism="$number_of_jobs" --disable-fetch-external --disable-ext-mysql-connector --disable-ccache --disable-gconf
--disable-online-update --disable-dependency-tracking --disable-sdremote-bluetooth --disable-kde --disable-kdeab
--disable-rpath --disable-gnome-vfs --disable-gstreamer --disable-zenity --disable-epm --disable-postgresql-sdbc
--disable-sdremote --disable-ext-report-builder --disable-ext-wiki-publisher --disable-scripting-beanshell
--without-gssapi --without-junit --without-unix-wrapper --enable-python=system
A few notes:
1. This was LibreOffice version 22.214.171.124.
2. I had a lot of stuff installed into /opt/<package name> and then lndir'd it up back to /opt (hence --with-external-hyph-dir is set to /opt/share/hyphen).
3. Because I don't do any external fetching and I never installed Python 3.3 and so I had to export a few variables before I started the build in order to use Python shipped with Slackware (and really old versions of Python might not work, mine was version 2.7 but I heard 2.5 is enough for LibreOffice).
a) export PYTHON=$(python -c "import sys, os; print os.path.join('python%s' % sys.version[:3])"
b) export PYTHON_CFLAGS=$($PYTHON-config --cflags)
c) export PYTHON_LIBS=$($PYTHON-config --libs)
4. You can actually leave --with-lang empty or not have it at all, I just chose to have a few other languages installed.
5. You need the help pack tarball (libreoffice-help-<VERSION>.tar.?z*), but if you don't want the help pack installed as a part of the install, then use --without-helppack-integration.
6. I had to grab: 1f24ab1d39f4a51faf22244c94a6203f-xmlsec1-1.2.14.tar.gz
and move it into /path/to/where/libreoffice/was/unpacked/libreoffice-126.96.36.199/src
(I thought xmlsec1 was a dependency so I tried installing it, but that didn't stop libreoffice from bugging me to move that tarball to the src/ directory :/ and LO still built without xmlsec1 installed anyways)
7. I also had to remove these images: /path/to/where/libreoffice/was/unpacked/libreoffice-188.8.131.52/vcl/qa/cppunit/graphicfilter/data/jpg/pass/CVE-2004-0200-?.jpg (where ? is the wildcard for a single character).
8. With the given configuration options I used, I needed these patches:
(Other configuration options may require other patches. According to #libreoffice-dev Google might not help, but using git log on the development branch of LibreOffice on the file that failed to make should).
9. After the build is done, you're going to have a lot of "gid_Module_<stuff>" files in your build root, but these can be removed (if you keep them you'll get these same files installed under / and I don't think you'd want that).
10. For cleaning up, I got a bunch of random crap in my /tmp (a few files ending in .tmp, 4 directories, ooopackaging, LibreOffice and LibreOffice_SDK and one more directory composed of random hexadecimal characters).