travis: Do our automake build tests with srcdir != builddir.

This will catch many automake bugs that end-users get to experience first,
otherwise.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

.travis.yml

@@ -591,7 +591,9 @@ script:
 
       export CFLAGS="$CFLAGS -isystem`pwd`";
 
-      ./autogen.sh --enable-debug
+      mkdir build &&
+      cd build &&
+      ../autogen.sh --enable-debug
         $LIBUNWIND_FLAGS
         $DRI_LOADERS
         --with-dri-drivers=$DRI_DRIVERS