MAINT: Refactor docker builds for platforms

Skip build tests if no docker available.

(There is no docker available for precise, so skip the builds for
precise).

Specify platform with env var, test 32 and 64 bit builds.

.travis.yml

@@ -6,23 +6,35 @@ cache:
   directories:
    - $HOME/.ccache
 
+env:
+  global:
+    - PLAT=x86_64
+
 matrix:
   include:
     - os: linux
       dist: precise
+    # 64-bit builds
+    - os: linux
+      dist: trusty
       env:
         - TEST_BUILDS=1
+      sudo: required
+    # 32-bit builds
     - os: linux
       dist: trusty
       env:
         - TEST_BUILDS=1
+        - PLAT=i686
       sudo: required
+    # Builds with caching
     - os: linux
       dist: trusty
       env:
         - TEST_BUILDS=1
         - USE_CCACHE=1
       sudo: required
+    # OSX builds
     - os: osx
       osx_image: xcode6.4
       env:

tests/test_multibuild.sh

@@ -18,12 +18,14 @@ else
     source tests/test_manylinux_utils.sh
 fi
 if [ -n "$TEST_BUILDS" ]; then
-    if [ -n "$IS_OSX" ] || [ ! -x "$(command -v docker)" ]; then
+    if [ -n "$IS_OSX" ]; then
         source tests/test_library_builders.sh
+    elif [ ! -x "$(command -v docker)" ]; then
+        echo "Skipping build tests; no docker available"
     else
         touch config.sh
         source travis_linux_steps.sh
-        build_multilinux i686 "source tests/test_library_builders.sh"
+        build_multilinux $PLAT "source tests/test_library_builders.sh"
     fi
 fi