BuildTools.jar is a solution to building Bukkit, CraftBukkit, Spigot, and the Spigot-API. All of which is done on your computer! A few prerequisite programs are necessary, but the instructions below will guide you through everything you need to do.
There are two applications necessary to use BuildTools: Git and Java.
In order for BuildTools to run on Windows, you will need to install Git. For Windows it is distributed via git-scm, which can be downloaded here. Install it where you like, it will provide git bash, which will be used to run the BuildTools jar. Just keep hitting next when running the installer.
Download JRE 8 from here and install. Just keep hitting next when running the installer.
Both git and Java, as well as util commands, can be installed using a single command via your package manager.
sudo apt-get install git openjdk-7-jre-headless tar
sudo dnf install git java-1.7.0-openjdk-devel tar
pacman -S jdk8-openjdk git
Git can be downloaded from: http://sourceforge.net/projects/git-osx-installer/files/
Java may need to be updated from the Apple distributed version, and even if previously updated, may need to be linked for shell use. Please follow steps found here: https://gist.github.com/johan/10590467
Download BuildTools.jar from https://hub.spigotmc.org/jenkins/job/BuildTools/lastSuccessfulBuild/artifact/target/BuildTools.jar.
Open your terminal if you are on Linux, or git bash on Windows.
Navigate to where you downloaded BuildTools.jar, or use the command line way to download the jar to your current directory.
Run BuildTools.jar from the terminal (Do not double-click BuildTools.jar) by doing the following:
Wait as it builds your jars. In a few minutes you should have freshly compiled jars!
You can find CraftBukkit and Spigot in the same directory you ran the the BuildTools.jar in (craftbukkit-1.10.jar and spigot-1.10.jar). You can find Spigot-API in \Spigot\Spigot-API\target\ (spigot-api-1.10-R0.1-SNAPSHOT.jar).