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

.travis.yml

@@ -26,6 +26,10 @@ matrix:
       compiler: clang
       os: linux
       dist: trusty
+    - env: COMPILER=none LINT=1
+      compiler: clang
+      os: linux
+      dist: trusty
     - env: PLATFORM=Unix COMPILER=g++-6
       compiler: gcc
       os: linux