Define "__GHCIDE__" on CPP

src/Development/IDE/GHC/CPP.hs

@@ -114,6 +114,8 @@ doCpp dflags raw input_fn output_fn = do
                     ++ map SysTools.Option sse_defs
                     ++ map SysTools.Option avx_defs
                     ++ mb_macro_include
+        -- Define a special macro "__GHCIDE__"
+                    ++ [ SysTools.Option "-D__GHCIDE__"]
         -- Set the language mode to assembler-with-cpp when preprocessing. This
         -- alleviates some of the C99 macro rules relating to whitespace and the hash
         -- operator, which we tend to abuse. Clang in particular is not very happy