Improve clangd support

clangd is a very useful tool, serving as a Language Server for C/C++,
helping non-IDE editors tremendously.
With these tweaks, it works flawlessly on my setup.

Add a .clangd config file to exclude the `-mno-direct-extern-access`
flag which works fine on gcc but not on clang.
Also tell CMake to export a thing called "compile commands", which is
essentially a JSON file detailing all the commands used in compilation,
so that clangd can correctly process headers and allow you to jump
between definitions, uses and whatnot.
Finally, adds `.cache/clangd` to the gitignore file.

Signed-off-by: tmpod <tom@tmpod.dev>

.clangd

@@ -0,0 +1,4 @@
+CompileFlags:
+  Remove:
+    # clang has an issue with this flag, while gcc does not
+    - "-mno-direct-extern-access"

.gitignore

@@ -188,3 +188,6 @@ screenshots/
 android/src/google/play/listings/
 keywords/
 iphone/metadata/**/keywords.txt
+
+# clangd cache
+.cache/clangd/

CMakeLists.txt

@@ -11,6 +11,8 @@ set(CMAKE_C_VISIBILITY_PRESET hidden)
 set(CMAKE_CXX_VISIBILITY_PRESET hidden)
 set(CMAKE_VISIBILITY_INLINES_HIDDEN ON)
 
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
 # Fixes warning ld: warning: ignoring duplicate libraries on Mac and Windows.
 if (POLICY CMP0156)
   cmake_policy(SET CMP0156 NEW)