Test common configure switches on travis.

Add a matrix build testing the most common options.

I don't see a way to also invoke Makefile.unix without
wrapping everything in a script, so leave that to jenkins
for now.

.travis.yml

 language: c
 
+env:
+  - CONFIG=""
+  - CONFIG="--enable-assertions"
+  - CONFIG="--enable-fixed-point"
+  - CONFIG="--enable-fixed-point --disable-float-api"
+  - CONFIG="--enable-fixed-point --enable-assertions"
+
 script:
   - ./autogen.sh
-  - ./configure
+  - ./configure $CONFIG
   - make distcheck