Add more junk & clangd symlinks to .gitignore

Ignore any root directory starting with build so that you can have many
build directories (build, build2, build-win32, ...) that get ignored.

Add .cache which clangd creates

Ignore compile_commands.json (and generate it by default) which helps
clangd, but needs to be symlinked to a compile_commands.json in a build
dir so isn't suitable for inclusion in the repo.

.gitignore

@@ -5,7 +5,7 @@
 *.o
 *.so
 
-build/
+/build*/
 **/__pycache__/**
 
 llarpd
@@ -18,6 +18,8 @@ CMakeFiles
 CMakeCache.txt
 .ninja_log
 .ninja_deps
+/.cache/
+/compile_commands.json
 
 callgrind.*
 .gdb_history

CMakeLists.txt

@@ -1,5 +1,7 @@
 cmake_minimum_required(VERSION 3.10)  # bionic's cmake version
 
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
 # Has to be set before `project()`, and ignored on non-macos:
 set(CMAKE_OSX_DEPLOYMENT_TARGET 10.12 CACHE STRING "macOS deployment target (Apple clang only)")