Generate .clangd in PROJECT_SOURCE_DIR (#1059)

This allows the project to be used as a submodule or subdirectory of
another CMake project without rewriting their `.clangd` file. This
includes using BT.CPP through CMake FetchContent or CPM.

Author: nyibbang

CMakeLists.txt

@@ -308,17 +308,17 @@ endif()
 
 ######################################################
 # Generate .clangd configuration file for standalone header checking
-file(WRITE ${CMAKE_SOURCE_DIR}/.clangd
+file(WRITE ${PROJECT_SOURCE_DIR}/.clangd
 "CompileFlags:
   Add:
     - -xc++
     - -std=c++17
-    - -I${CMAKE_SOURCE_DIR}/include
-    - -I${CMAKE_SOURCE_DIR}/3rdparty
-    - -I${CMAKE_SOURCE_DIR}/3rdparty/minitrace
-    - -I${CMAKE_SOURCE_DIR}/3rdparty/tinyxml2
-    - -I${CMAKE_SOURCE_DIR}/3rdparty/minicoro
-    - -I${CMAKE_SOURCE_DIR}/3rdparty/lexy/include
+    - -I${PROJECT_SOURCE_DIR}/include
+    - -I${PROJECT_SOURCE_DIR}/3rdparty
+    - -I${PROJECT_SOURCE_DIR}/3rdparty/minitrace
+    - -I${PROJECT_SOURCE_DIR}/3rdparty/tinyxml2
+    - -I${PROJECT_SOURCE_DIR}/3rdparty/minicoro
+    - -I${PROJECT_SOURCE_DIR}/3rdparty/lexy/include
 ")
 
 ######################################################