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You will need: Qt > 4.6 (NOT 4.7 - broken), gnu make >= 3.81, recent gcc which is able to compile Qt stuff (3.4 and 4.3,4.5.3 worked fine me). See notes in Makefile and files in doc/.Good luck.
Windows build is currently done totally on linux: it uses mingw, windows SDK archive and wine to run installer creation script. To build exe, use make CROSS_BUILD=yes all command in source directory.
$ hg clone http://mentat.hg.sourceforge.net:8000/hgroot/mentat/mentat OR $ tar xvzf mentat-src-(version).tar.bz2
$ cd mentat/ $ make all (default prefix is /usr/local/) OR $ make prefix=/some/path all
(as root) $ make install
Currently I have no time/will to build packages for each distribution. It looks like OpenSuse build service will provide everything required, but to write correct spec I need to install some distribution - I have no idea how Qt is packaged and called in different distros. Actually, there is a spec in sources that should work for Fedora/OpenSuse of recent versions.
To enable debug everywhere, set GLOBAL_DEBUG=yes in build/makefile. To get debugging output on windows, set DEBUG_CONSOLE=yes. This will give one more window with all debug output going into it. On linux debug just goes to stdout, to have debug on stdout in windows, you will need to change build targets to commad-line executable.
To enable per-module debug, edit module.mk in the directory of interest and set DEBUG=yes (after default one). Each file has debug_enable define in the head, which will turn on functions calls tracing for this file.