Prevent compilation in source dir.

Only allows compilation is top-src-dir/build

.gitignore

 *~
 build/
+CMakeFiles/
+CMakeCache.txt

CMakeLists.txt

-cmake_minimum_required (VERSION 2.6)
+cmake_minimum_required (VERSION 3.4)
+
+if (${CMAKE_BINARY_DIR} MATCHES "^${CMAKE_SOURCE_DIR}")
+  if (NOT ${CMAKE_BINARY_DIR} MATCHES "^${CMAKE_SOURCE_DIR}/build")
+    message("")
+    message("##############################################")
+    message("     In-source building is not allowed!       ")
+    message("##############################################")
+    message("")
+    message(" Run cmake outside from source directory ")
+    message(" or from ${CMAKE_SOURCE_DIR}/build")
+    message("")
+    message("----------------------------------------------")
+    message(" warning: remaining generated files!")
+    message("   ${CMAKE_BINARY_DIR}/CMakeCache.txt")
+    message("   ${CMAKE_BINARY_DIR}/CMakeFiles")
+    message("----------------------------------------------")
+    message("")
+    message(" Please remove remaining generated files ")
+    message(" and run cmake from an appropriate location")
+    message("")
+
+    message(FATAL_ERROR  "\n** CMake aborted **")
+  endif()
+endif()
+
 project (Pastis)
 add_executable(pastis main.cpp)