Add a bindep.

Add a bindep file to fix the rsvg-convert command.

Change-Id: I1c8f252dae5cffec5f98a22274240edb60b2504d

bindep.txt

@@ -0,0 +1,2 @@
+librsvg2-tools [doc platform:rpm]
+librsvg2-bin [doc platform:dpkg]

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 2.0
 skipsdist = True
-envlist = docs,pep8
+envlist = docs,pep8,bindep
 
 [testenv]
 usedevelop = True
@@ -52,3 +52,15 @@ basepython = python3
 deps = {[testenv:docs]deps}
 
 commands = sphinx-build -a -E -W -d deploy-guide/build/doctrees -b html deploy-guide/source deploy-guide/build/html
+
+[testenv:bindep]
+# Do not install any requirements. We want this to be fast and work even if
+# system dependencies are missing, since it's used to tell you what system
+# dependencies are missing! This also means that bindep must be installed
+# separately, outside of the requirements files, and develop mode disabled
+# explicitly to avoid unnecessarily installing the checked-out repo too (this
+# further relies on "tox.skipsdist = True" above).
+usedevelop = False
+deps = bindep
+commands =
+  bindep test