Update .gitignore to exclude non out-of-source build trees from Clion.

Clion 2016.3 will by default generate build tree inside the project's root directory, which is also also our source tree.
[ci skip]

.gitignore

@@ -51,6 +51,7 @@ generated/
 
 # JetBrains IDE project settings
 /.idea/
+/cmake-build-*/
 
 # Code::Blocks project settings
 /*.cbp