This guide explains how to build Krita from standard Qt 5 and KDE Frameworks 5 development packages provided with Debian/Ubuntu. Building Krita is fundamentally similiar to building Calligra, but there are a few differences. I am too lazy to copy all of those instructions into this page, it would be nice if you did it though. =)
The first important difference is that Krita's dependencies are different. Drumroll, please:
sudo apt install extra-cmake-modules libkf5archive-dev libkf5completion-dev libkf5config-dev libkf5coreaddons-dev libkf5guiaddons-dev libkf5i18n-dev libkf5itemmodels-dev libkf5itemviews-dev libkf5widgetsaddons-dev libkf5windowsystem-dev libkf5kiocore5 qtbase5-dev libqt5svg5-dev qtdeclarative5-dev libqt5x11extras5-dev libqt5opengl5-dev libeigen3-dev libxi-dev libboost-all-dev libopenexr-dev vc-dev libexiv2-dev libgsl0-dev liblcms2-dev libpoppler-qt5-dev shared-mime-info
Technically you can get away without each and every one. KIO is for optional thumbnail integration with Dilphing. QtDeclarative is for Krita Sketch. Poppler is for PDF import. GSL is for the transform tool and eventually the magnetic lasso tool. Vc is for faster SIMD accelerated painting. SharedMimeInfo is for determining file types. If you want to build Krita without these features, you can omit the package.
Arch Linux dependencies
Building Krita 3.x on Arch requires slightly more dependencies than running 2.9.9; this should get you going (90% of these are 2.9.9 dependencies already):
sudo pacman -S attica-qt4 qca-qt4 libdbusmenu-qt4 polkit-qt4 exiv2 strigi docbook-xml docbook-xsl libplist libusbmuxd libimobiledevice upower libatasmart udisks2 cdparanoia libvisual gst-plugins-base phonon-qt4-gstreamer libqzeitgeist phonon-qt4 media-player-info qtwebkit kdelibs libakonadi-qt4 libical libdmtx qrencode prison qjson kdepimlibs4 libkactivities4 calligra-libs librevenge libwpd libodfgen calligra-filters libspnav calligra-plugins qt5-xmlpatterns qt5-declarative solid karchive gamin kcoreaddons kwidgetsaddons qt5-x11extras kjobwidgets kconfig kwindowsystem kcrash kdbusaddons kglobalaccel kcompletion qt5-script ki18n kservice qt5-svg polkit-qt5 kauth kcodecs kguiaddons kconfigwidgets kitemviews kiconthemes sonnet ktextwidgets attica-qt5 kxmlgui kbookmarks phonon-qt5-gstreamer phonon-qt5 libdbusmenu-qt5 knotifications gpgmepp kwallet kio kpackage kdeclarative kcmutils kactivities-frameworks sound-theme-freedesktop libcanberra oxygen-icons kdebase-runtime opencolorio libraw libkdcraw fftw gsl
The second important difference is that Krita's source repository is different.
cd ~/kf5/src (Or, wherever you want to download the source code) git clone kde:krita
The third important difference is that Krita's build system has diverged. For example, since there is only one product, Krita does not use productsets any longer. This will turn of building tests so that building is faster. RelWithDebInfo optimization level is nearly always good enough for everyday debugging, but it is also fast enough to paint with. =)
cd ~/kf5/build/krita cmake -DCMAKE_INSTALL_PREFIX=$HOME/kde/inst5 $HOME/kde/src/calligra -DCMAKE_BUILD_TYPE=RelWithDebInfo -DBUILD_TESTING=OFF make -j12
Remember that you can change build settings with cmake . -DNEW_SETTING or by using cmake-gui found in the package cmake-qt-gui
XDG_DATA_DIRS. If you are OK installing in your home folder, with -DCMAKE_INSTALL_PREFIX=~/, you need only one environment variable to run Krita:
KRITA_PLUGIN_PATH. Currently, running inside a debugger can be a bit fragile due to plugin loading. Krita's plugins are built in several different paths inside the source directory, and the installation process copies them into a single folder. However the debugger might accidentally not catch onto the plugin installation directory. (For example, if it thinks the plugin path is ./build/krita/plugins/ then it will miss the plugins contained in ./build/plugins/. It may be possible in the future to fix this by altering Krita's codebase, but such large changes are not yet possible until the break with Calligra repo is more complete.)
You can force Krita to look in a specific folder for plugins using the environment variable KRITA_PLUGIN_PATH. For example, using Kubuntu and installing Krita in ~/, I can use a debugger if I set the environment variable: