Only run builds for subset of matrix

.travis.yml

+# Set TEST_BUILDS=1 to run library_builders tests
+
 language: generic
 
 matrix:
   include:
     - os: linux
+      dist: precise
+      env:
+        - TEST_BUILDS=1
+    - os: linux
+      dist: trusty
+      env:
+        - TEST_BUILDS=1
     - os: osx
       osx_image: xcode6.4
       env:
         - PYTHON_VERSION=2.6
+        - TEST_BUILDS=1
     - os: osx
       osx_image: xcode6.4
       env:
@@ -15,6 +25,7 @@ matrix:
     - os: osx
       env:
         - PYTHON_VERSION=2.7.11
+        - TEST_BUILDS=1
     - os: osx
       language: objective-c
       env:

tests/test_multibuild.sh

@@ -14,7 +14,9 @@ else
     source manylinux_utils.sh
     source tests/test_manylinux_utils.sh
 fi
-source tests/test_library_builders.sh
+if [ -n "$TEST_BUILDS" ]; then
+    source tests/test_library_builders.sh
+fi
 
 # Exit 1 if any test errors
 barf