Disallow in-source build (build-root is directory with top-level CMakeLists.txt)

Author: Growl1234

CMakeLists.txt

@@ -1,4 +1,14 @@
 cmake_minimum_required(VERSION 3.16)
+
+# Require out-of-source builds
+file(TO_CMAKE_PATH "${CMAKE_BINARY_DIR}/CMakeLists.txt" LOC_PATH)
+if(EXISTS "${LOC_PATH}")
+  message(
+    FATAL_ERROR
+      "You cannot build in a source directory (or any directory with a CMakeLists.txt file). "
+      "Please make a build subdirectory.")
+endif()
+
 include(CMakeDependentOption)
 if(POLICY CMP0135) # https://cmake.org/cmake/help/git-stage/policy/CMP0135.html
   cmake_policy(SET CMP0135 NEW)