set MAKEFLAGS to use multiple processors on Travis CI
diff --git a/travis.sh b/travis.sh
index bd226a8..3a6c297 100755
--- a/travis.sh
+++ b/travis.sh
@@ -1,5 +1,16 @@
#!/usr/bin/env sh
set -evx
+
+# if possible, ask for the precise number of processors,
+# otherwise take 2 processors as reasonable default; see
+# https://docs.travis-ci.com/user/speeding-up-the-build/#Makefile-optimization
+if [ -x /usr/bin/getconf ]; then
+ MAKEFLAGS=j$(/usr/bin/getconf _NPROCESSORS_ONLN)
+else
+ MAKEFLAGS="j2"
+fi
+export MAKEFLAGS
+
env | sort
mkdir build || true