Auto-disable (deep) caching of core builds if needed (#479)

When build-path and cache-path are on different partitions it will not
be possible (on windows) to reference them with a relative path.
This commit allows this by automatically disabling the aggressive core
caching.

Previously an error was emitted and the build interrupted.

Author: cmaglie

Diff for: legacy/builder/phases/core_builder.go

@@ -53,7 +53,13 @@ func (s *CoreBuilder) Run(ctx *types.Context) error {
 	}
 
 	if coreBuildCachePath != nil {
-		if err := coreBuildCachePath.MkdirAll(); err != nil {
+		if _, err := coreBuildCachePath.RelTo(ctx.BuildPath); err != nil {
+			logger := ctx.GetLogger()
+			logger.Println(constants.LOG_LEVEL_INFO, "Couldn't deeply cache core build: {0}", err)
+			logger.Println(constants.LOG_LEVEL_INFO, "Running normal build of the core...")
+			coreBuildCachePath = nil
+			ctx.CoreBuildCachePath = nil
+		} else if err := coreBuildCachePath.MkdirAll(); err != nil {
 			return i18n.WrapError(err)
 		}
 	}