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

3 files changed, 49 insertions, 2 deletions

diff --git a/util/travis/before_install.sh b/util/travis/before_install.sh
index ea85b3db6..a4328fa46 100755
--- a/util/travis/before_install.sh
+++ b/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
diff --git a/util/travis/common.sh b/util/travis/common.sh
index 16c7db324..35ceec08d 100644
--- a/util/travis/common.sh
+++ b/util/travis/common.sh
@@ -1,8 +1,9 @@
 #!/bin/bash -e
 
 # Relative to git-repository root:
-TRIGGER_COMPILE_PATHS="src/|CMakeLists.txt|cmake/Modules/|util/travis/|util/buildbot/"
+TRIGGER_COMPILE_PATHS="src/.*\.(c|cpp|h)|CMakeLists.txt|cmake/Modules/|util/travis/|util/buildbot/"
 
 needs_compile() {
 	git diff --name-only $TRAVIS_COMMIT_RANGE | egrep -q "^($TRIGGER_COMPILE_PATHS)"
 }
+
diff --git a/util/travis/script.sh b/util/travis/script.sh
index 84ea578a5..557822e1f 100755
--- a/util/travis/script.sh
+++ b/util/travis/script.sh
@@ -3,21 +3,57 @@
 
 needs_compile || exit 0
 
+function perform_lint() {
+	CLANG_FORMAT=clang-format-3.9
+	if [ "$TRAVIS_EVENT_TYPE" = "pull_request" ]; then
+		# Get list of every file modified in this pull request
+		files_to_lint="$(git diff --name-only --diff-filter=ACMRTUXB $TRAVIS_COMMIT_RANGE | grep '^src/[^.]*[.]\(cpp\|h\)$' | egrep -v '^src/(gmp|lua|jsoncpp)/' || true)"
+	else
+		# Check everything for branch pushes
+		files_to_lint="$(find src/ -name '*.cpp' -or -name '*.h' | egrep -v '^src/(gmp|lua|jsoncpp)/')"
+	fi
+
+	local fail=0
+	for f in ${files_to_lint}; do
+		d=$(diff -u "$f" <(${CLANG_FORMAT} "$f") || true)
+		if ! [ -z "$d" ]; then
+			printf "The file %s is not compliant with the coding style:\n%s\n" "$f" "$d"
+			# Disable build failure at this moment as we need to have a complete MT source whitelist to check
+			fail=0
+		fi
+	done
+
+	if [ "$fail" = 1 ]; then
+		exit 1
+	fi
+
+	exit 0
+}
+
+if [[ "$LINT" == "1" ]]; then
+	# Lint with exit CI
+	perform_lint
+fi
+
 if [[ $PLATFORM == "Unix" ]]; then
 	mkdir -p travisbuild
 	cd travisbuild || exit 1
+
 	CMAKE_FLAGS=''
 	if [[ $COMPILER == "g++-6" ]]; then
 		export CC=gcc-6
 		export CXX=g++-6
 	fi
+
 	# Clang builds with FreeType fail on Travis
 	if [[ $CC == "clang" ]]; then
 		CMAKE_FLAGS+=' -DENABLE_FREETYPE=FALSE'
 	fi
+
 	if [[ $TRAVIS_OS_NAME == "osx" ]]; then
 		CMAKE_FLAGS+=' -DCUSTOM_GETTEXT_PATH=/usr/local/opt/gettext'
 	fi
+
 	cmake -DCMAKE_BUILD_TYPE=Debug \
 		-DRUN_IN_PLACE=TRUE \
 		-DENABLE_GETTEXT=TRUE \