build: prevent in-tree builds for the time being, as it's not supported

CMakeLists.txt

@@ -8,6 +8,9 @@ endif()
 # Point CMake at any custom modules we may ship
 list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake")
 
+# We don't support building in-tree.
+include(PreventInTreeBuilds)
+
 # Prefer our bundled versions of dependencies.
 set(DEPS_PREFIX "${CMAKE_CURRENT_SOURCE_DIR}/.deps/usr" CACHE PATH "Path prefix for finding dependencies")
 if(CMAKE_CROSSCOMPILING AND NOT UNIX)

cmake/PreventInTreeBuilds.cmake

@@ -0,0 +1,23 @@
+function(PreventInTreeBuilds)
+  get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" REALPATH)
+  get_filename_component(bindir "${CMAKE_BINARY_DIR}" REALPATH)
+
+  if("${srcdir}" STREQUAL "${bindir}")
+    message("")
+    message("!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!")
+    message("Neovim doesn't support in-tree builds.  It's recommended that you")
+    message("use a build/ subdirectory:")
+    message("    mkdir build")
+    message("    cd build")
+    message("    cmake <OPTIONS> ..")
+    message("")
+    message("Make sure to cleanup some CMake artifacts from this failed build")
+    message("with:")
+    message("    rm -rf CMakeFiles CMakeCache.txt")
+    message("!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!")
+    message("")
+    message(FATAL_ERROR "Stopping build.")
+  endif()
+endfunction()
+
+PreventInTreeBuilds()