Travis: Only trigger a compile, when C/C++ related files were touched (#4284)

util/travis/before_install.sh

 #!/bin/bash -e
+echo "Preparing for $TRAVIS_COMMIT_RANGE"
+. util/travis/common.sh
+
+needs_compile || exit 0
 
 if [[ $TRAVIS_OS_NAME == "linux" ]]; then
 	sudo apt-get update

util/travis/common.sh

+#!/bin/bash -e
+
+# Relative to git-repository root:
+TRIGGER_COMPILE_PATHS="src/|CMakeLists.txt|cmake/Modules/|util/travis/|util/buildbot/"
+
+needs_compile() {
+	git diff --name-only $TRAVIS_COMMIT_RANGE | egrep -q "^($TRIGGER_COMPILE_PATHS)"
+}

util/travis/script.sh

 #!/bin/bash -e
+. util/travis/common.sh
+
+needs_compile || exit 0
 
 if [[ $PLATFORM == "Unix" ]]; then
 	mkdir -p travisbuild