added linux build instructions, Vagrantfile to prove it, and updated .gitignore

.gitignore

@@ -78,3 +78,6 @@ apple/RetroArch_iOS.xcodeproj/project.xcworkspace/*
 
 apple/iOS/build/
 apple/iOS/modules/
+
+obj-unix/
+.vagrant/

README.md

@@ -110,6 +110,17 @@ Users only need to configure a certain option if the desired value deviates from
 To configure joypads, use the built-in menu or the <tt>retroarch-joyconfig</tt> command-line tool.
 
 ## Compiling and installing
+<b>Linux</b><br/>
+- Prerequisites:
+```bash
+sudo apt-get install -y make git-core curl g++ pkg-config libglu1-mesa-dev freeglut3-dev mesa-common-dev libsdl1.2-dev libsdl-image1.2-dev libsdl-mixer1.2-dev libsdl-ttf2.0-dev
+```
+- ```bash
+./configure
+```
+- ```bash
+make
+```
 
 <b>Mac</b><br/>
 - Prerequisites: [XCode](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB4QFjAA&url=https%3A%2F%2Fitunes.apple.com%2Fus%2Fapp%2Fxcode%2Fid497799835%3Fmt%3D12&ei=ZmfeVNPtIILVoASBnoCYBw&usg=AFQjCNGrxKmVtXUdvUU3MhqZhP4MHT6Gtg&sig2=RIXKsWQ79YTQBt_lK5fdKA&bvm=bv.85970519,d.cGU), [Cg](https://developer.nvidia.com/cg-toolkit-download).

Vagrantfile

@@ -0,0 +1,20 @@
+Vagrant.configure("2") do |config|
+  config.vm.box = "ubuntu/trusty64"
+  config.vm.provision :shell, :inline => $BOOTSTRAP_SCRIPT # see below
+end
+
+$BOOTSTRAP_SCRIPT = <<EOF
+  set -e # Stop on any error
+
+  export DEBIAN_FRONTEND=noninteractive
+
+  echo VAGRANT SETUP: DOING APT-GET STUFF...
+  # Install prereqs
+  sudo apt-get update
+  sudo apt-get install -y make git-core curl g++ pkg-config libglu1-mesa-dev freeglut3-dev mesa-common-dev libsdl1.2-dev libsdl-image1.2-dev libsdl-mixer1.2-dev libsdl-ttf2.0-dev
+
+  # Make vagrant automatically go to /vagrant when we ssh in.
+  echo "cd /vagrant" | sudo tee -a ~vagrant/.profile
+
+  echo VAGRANT IS READY.
+EOF