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Install dependencies. Here's an example for Debian/Ubuntu:
-$ sudo apt-get install build-essential libirrlicht-dev cmake libbz2-dev libpng12-dev libjpeg-dev libxxf86vm-dev libgl1-mesa-dev libsqlite3-dev libogg-dev libvorbis-dev libopenal-dev libcurl4-gnutls-dev libfreetype6-dev zlib1g-dev libjsoncpp-dev
+$ sudo apt-get install build-essential libirrlicht-dev cmake libbz2-dev libpng12-dev libjpeg-dev libxxf86vm-dev libgl1-mesa-dev libsqlite3-dev libogg-dev libvorbis-dev libopenal-dev libcurl4-gnutls-dev libfreetype6-dev zlib1g-dev libgmp-dev libjsoncpp-dev
You can install git for easily keeping your copy up to date.
If you dont want git, read below on how to get the source without git.
ENABLE_REDIS - Build with libhiredis; Enables use of Redis map backend
ENABLE_SOUND - Build with OpenAL, libogg & libvorbis; in-game Sounds
ENABLE_LUAJIT - Build with LuaJIT (much faster than non-JIT Lua)
+ENABLE_SYSTEM_GMP - Use GMP from system (much faster than bundled mini-gmp)
RUN_IN_PLACE - Create a portable install (worlds, settings etc. in current directory)
USE_GPROF - Enable profiling using GProf
VERSION_EXTRA - Text to append to version (e.g. VERSION_EXTRA=foobar -> Minetest 0.4.9-foobar)