[cmake] Clean up rpath settings

Only build with install path if RELOCATABLE is on, otherwise we can stick to
the default build rpath.

We were setting these options to their default values:
CMAKE_MACOS_RPATH, CMAKE_SKIP_BUILD_RPATH, CMAKE_SKIP_INSTALL_RPATH

CMakeLists.txt

@@ -158,17 +158,11 @@ endforeach()
 
 include(ExternalProject)
 
-
-# CMAKE_BUILD_WITH_INSTALL_RPATH should be set to true.
-# It is because `nekotools boot` will use the neko in build dir during build.
-set(CMAKE_BUILD_WITH_INSTALL_RPATH TRUE)
-
 if (RELOCATABLE)
-	# https://cmake.org/Wiki/CMake_RPATH_handling
-	set(CMAKE_SKIP_BUILD_RPATH FALSE)
-	set(CMAKE_SKIP_INSTALL_RPATH FALSE)
+	# https://gitlab.kitware.com/cmake/community/-/wikis/doc/cmake/RPATH-handling
+	# Set this to true, otherwise the binaries won't be relocatable until after installation:
+	set(CMAKE_BUILD_WITH_INSTALL_RPATH TRUE)
 	if(APPLE)
-		set(CMAKE_MACOSX_RPATH TRUE)
 		set(CMAKE_INSTALL_RPATH @executable_path/)
 	elseif(UNIX)
 		set(CMAKE_INSTALL_RPATH \$ORIGIN)