CMake: prefer the vendored simdjson include path

On macOS, locating DevIL via Homebrew brings /opt/homebrew/include onto
the global include path. A Homebrew-installed simdjson there can shadow
our vendored, version-matched copy in rts/lib/simdjson/include and break
the build (a newer simdjson trips an AppleClang macro collision).

Add BEFORE to the include_directories() call so the vendored simdjson
path always leads the include search and <simdjson.h> resolves in-tree
on every platform. This only reorders the vendored dir to the front and
is a no-op where no Homebrew simdjson is present.

Source: ExaDev/RecoilEngine 806d539.

Author: tomjn

rts/CMakeLists.txt

@@ -142,7 +142,12 @@ make_global_var(engineSources
 		${sources_engine_ExternalAI}
 	)
 include_directories(${CMAKE_CURRENT_SOURCE_DIR}/lib/assimp/include) #FIXME: hack for rts/Rendering/Models/IModelParser.cpp
-include_directories(${CMAKE_CURRENT_SOURCE_DIR}/lib/simdjson/include)
+# On macOS, locating DevIL via Homebrew pulls /opt/homebrew/include onto the
+# global include path, where a transitively-installed Homebrew simdjson.h can
+# shadow our vendored, version-matched copy and break the build (newer simdjson
+# trips an AppleClang macro collision). Prepend so <simdjson.h> always resolves
+# to the in-tree version on every platform.
+include_directories(BEFORE ${CMAKE_CURRENT_SOURCE_DIR}/lib/simdjson/include)
 include_directories(${CMAKE_CURRENT_SOURCE_DIR}/lib/fastgltf/include)
 
 ### Add headers for generated project files (e.g. Code::Blocks)