build system: removed the in-source build check

CMakeLists.txt

@@ -4,14 +4,6 @@
 
 cmake_minimum_required(VERSION 3.12.4)
 
-#
-# Building in-tree is not allowed (we take care of your craziness).
-#
-
-if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_BINARY_DIR})
-    message(FATAL_ERROR "Prevented in-tree built. Please create a build directory outside of the source code and call cmake from there. Thank you.")
-endif()
-
 #
 # Read project version
 #