Installation.md: add dependencies section.

.travis.yml

@@ -1,6 +1,10 @@
 os:
   - linux
   - osx
+addons:
+  apt:
+    packages:
+    - bsdmainutils
 script:
   - ./check_integrity.sh $(find bin | cut -b 5- | xargs)
   # Test for the brew release

Installation.md

@@ -1,5 +1,14 @@
 # Installing git-extras
 
+## Dependencies
+
+Some commands require extra dependencies which are unavailable in some platforms.
+You may need to install them manually.
+
+Those dependencies are listed below:
+
+* column
+
 ## Installing with a package manager
 
 ### Debian
@@ -84,6 +93,8 @@ Use the instructions to build from source below. Make sure you are using `gmake`
 
 ## Building from source
 
+Read [Dependencies](#dependencies) and ensure they are installed.
+
 Obtain the git-extras source by cloning [its GitHub repo](https://github.com/tj/git-extras.git) or downloading a tarball of a [release](https://github.com/tj/git-extras/releases). Then install it by doing `make install` from the source tree.
 
 ```bash

Makefile

@@ -25,7 +25,12 @@ default: install
 
 docs: $(MAN_HTML) $(MAN_PAGES)
 
-install:
+check:
+	@echo "Check dependencies before installation"
+	@./check_dependencies.sh
+	@echo
+
+install: check
 	@if [ "$(INSTALL_VIA)" = brew ]; then \
 		git apply brew-release.patch || { echo "Can't apply brew release patch"; exit 1; } \
 	fi
@@ -120,4 +125,4 @@ docclean:
 	rm -f man/*.1
 	rm -f man/*.html
 
-.PHONY: default docs clean docclean install uninstall
+.PHONY: default docs clean docclean check install uninstall

check_dependencies.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+err() {
+    echo >&2 "$1"
+    exit 1
+}
+
+if ! test "$(which column)"; then
+    err "Need to install dependency 'column' before installation"
+fi