travis: verify out-of-tree builds & dist targets

scripts/travis-build.sh

 #!/bin/sh
 
 set -ex
 
 setup_env() {
   # Travis sets CC/CXX to the system toolchain, so our .travis.yml
   # exports USE_{CC,CXX} for this script to use.
   if [ -n "$USE_CC" ]; then
       export CC=$USE_CC
   fi
   if [ -n "$USE_CXX" ]; then
       export CXX=$USE_CXX
   fi
   # Use -jN for faster builds. Travis build machines under Docker
   # have a lot of cores, but are memory-limited, so the kernel
   # will OOM if we try to use them all, so use at most 4.
   # See https://github.com/travis-ci/travis-ci/issues/1972
   export NCPUS=$(getconf _NPROCESSORS_ONLN)
   export JOBS=$(( $NCPUS < 4 ? $NCPUS : 4 ))
 }
 
+# Do an in-tree build and make sure tests pass.
 build() {
   ./configure
   make -j${JOBS} check VERBOSE=1
+  make distclean
+}
+
+# Do an out-of-tree build and make sure we can create a release tarball.
+build_out_of_tree() {
+  mkdir -p build/native
+  cd build/native
+  ../../configure
+  make -j${JOBS} distcheck VERBOSE=1
 }
 
 main() {
   setup_env
   build
+  build_out_of_tree
 }
 
 main "$@"