counsel.el (counsel-compile): don't squash default-directory

We may not have defined a blddir, especially if the user has manually
entered the make invocation. If blddir is nil then just keep
default-directory as is.

counsel.el

@@ -5414,7 +5414,7 @@ specified by the `blddir' property."
       (when (get-char-property 0 'cmd cmd)
         (setq cmd (substring-no-properties
                    cmd 0 (next-single-property-change 0 'cmd cmd))))
-      (let ((default-directory blddir)
+      (let ((default-directory (or blddir default-directory))
             (compilation-environment bldenv))
         ;; No need to specify `:history' because of this hook.
         (add-hook 'compilation-start-hook #'counsel-compile--update-history)