CMake: make sure compile_commands.json is readable by ccls

CMakeLists.txt

@@ -4,6 +4,15 @@ project(gromit-mpx LANGUAGES C)
 set(target_name gromit-mpx)
 set(version 1.3)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+if(CMAKE_GENERATOR MATCHES "Unix Makefiles|Ninja")
+# some LSP servers expect compile_commands.json in the project root
+add_custom_target(
+    copy-compile-commands ALL
+    ${CMAKE_COMMAND} -E copy_if_different
+        ${CMAKE_BINARY_DIR}/compile_commands.json
+        ${CMAKE_CURRENT_SOURCE_DIR}
+    )
+endif(CMAKE_GENERATOR MATCHES "Unix Makefiles|Ninja")
 
 set(CMAKE_C_FLAGS  " ${CMAKE_C_FLAGS} -Wall -Wextra -Wno-unused-parameter")