Uses system provided libgmp if found, if not it falls back to mini-gmp.

On Debian 8...
```
Package libjpeg8-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  libjpeg62-turbo-dev
```
Using libjpeg-dev should make the OS select the most appropriate lib to be installed.
These are the packages available on Debian 8.
```
libjpeg-dev - Development files for the JPEG library [dummy package]
libjpeg-turbo-progs - Programs for manipulating JPEG files
libjpeg-turbo-progs-dbg - Programs for manipulating JPEG files (debugging symbols)
libjpeg62-turbo - libjpeg-turbo JPEG runtime library
libjpeg62-turbo-dbg - Debugging symbols for the libjpeg-turbo JPEG library
libjpeg62-turbo-dev - Development files for the libjpeg-turbo JPEG library
libturbojpeg1 - TurboJPEG runtime library - SIMD optimized
libturbojpeg1-dbg - TurboJPEG runtime library - SIMD optimized (debugging symbols)
libturbojpeg1-dev - Development files for the TurboJPEG library

```
This seems to work on all current Debian versions, Wheezy, Jessie and Sid.
https://packages.debian.org/wheezy/libjpeg-dev
https://packages.debian.org/jessie/libjpeg-dev
https://packages.debian.org/stretch/libjpeg-dev

Im not sure if Ubuntu has this dummy package.

http://packages.ubuntu.com/search?keywords=libjpeg-dev

Perhaps this should be further discussed.

libjpeg8-dev -> libjpeg-dev

  * Combine client and server man pages.
  * Update unit test options and available databases in man page.
  * Add `--worldname` to man page.
  * Fix a bunch of places where `"Minetest"` was used directly instead of `PROJECT_NAME`.
  * Disable server build by default on all operating systems.
  * Make `ENABLE_FREETYPE` not fail if FreeType isn't found.
  * Enable LevelDB, Redis, and FreeType detection by default.
  * Remove the `VERSION_PATCH_ORIG` hack.
  * Add option to search for and use system JSONCPP.
  * Remove broken LuaJIT version detection.
  * Rename `DISABLE_LUAJIT` to `ENABLE_LUAJIT`.
  * Rename `minetest_*` variables in `version.{h,cpp}` to `g_*`.
  * Clean up style of CMake files.

* Remove --enable-unittests and --disable-unittests and add --do-unittests function
* --do-unittests function will exit 0 on success.
* minetest and minetestserver binaries are launched with --do-unittests in travis build.