Add clang format & skip build if no source file modified (#5433)

* [BUILD] Add clang format + build skipping

* Add clang-format tool to check codestyle.
Warning: it check the whole modified file, not the diff part, it's why it's lazy. Please also look if rules are perfect, i take the Linux codestyle from LLVM site

Fix issue #5415

* Skip building project if no file is modified
* Fix a wrong brace to trigger LINT

* Make lint step outside of unix build scope

* Add AccessModifierOffset: -8

* Typo fix & needs compile fix

* Fix header priorities

util/travis/before_install.sh

@@ -1,7 +1,15 @@
 #!/bin/bash -e
 echo "Preparing for $TRAVIS_COMMIT_RANGE"
-. util/travis/common.sh
 
+if [[ "$LINT" == "1" ]]; then
+	curl http://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+	sudo add-apt-repository "deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-3.9 main"
+	sudo apt-get -yq update
+	sudo apt-get install clang-format-3.9
+	exit 0
+fi
+
+. util/travis/common.sh
 needs_compile || exit 0
 
 if [[ $TRAVIS_OS_NAME == "linux" ]]; then
@@ -11,6 +19,7 @@ fi
 
 if [[ $PLATFORM == "Unix" ]]; then
 	if [[ $TRAVIS_OS_NAME == "linux" ]]; then
+		sudo apt-get update
 		sudo apt-get install libirrlicht-dev cmake libbz2-dev libpng12-dev \
 			libjpeg-dev libxxf86vm-dev libgl1-mesa-dev libsqlite3-dev \
 			libhiredis-dev libogg-dev libgmp-dev libvorbis-dev libopenal-dev \
@@ -21,6 +30,7 @@ if [[ $PLATFORM == "Unix" ]]; then
 		if [[ "$VALGRIND" == "1" ]]; then
 			sudo apt-get install valgrind
 		fi
+
 	else
 		brew update
 		brew install freetype gettext hiredis irrlicht jpeg leveldb libogg libvorbis luajit