Add requirements and document defaults, add matrix

Default platform is linux and gcc, maybe osx later.

Unfortunately can't test things like BSD / Cygwin.

Make sure to include requirements for all defaults.

Don't bother checking the extras like gnome or gtk.

Extra builds to check:
* --with-auth (PR distcc#215)
* CC=gcc-7 (PR distcc#207)
* CC=musl-gcc (PR distcc#211)

Closes distcc#222.

.travis.yml

@@ -5,9 +5,22 @@ addons:
         packages:
             - python3-dev
             - libiberty-dev
+            - libavahi-client-dev
+
+dist: trusty
+
+os: linux
+
+compiler: gcc
+
+matrix:
+    include:
+        - env: "OPTIONS=--with-auth"
+        - compiler: gcc-7
+        - compiler: musl-gcc
 
 before_script: ./autogen.sh
 
 script:
-    - ./configure
+    - ./configure $OPTIONS
     - make check