Introduce flycheck-mode-line-lighter-base and use it, with test

flycheck.el

@@ -888,6 +888,18 @@ Set this variable to nil to disable the mode line completely."
   :risky t
   :package-version '(flycheck . "0.20"))
 
+(defcustom flycheck-mode-line-prefix "FlyC"
+  "Base mode line lighter for Flycheck.
+
+This will have an effect only with the default
+`flycheck-mode-line'.
+
+If you've customized `flycheck-mode-line' then the customized
+function must be updated to use this variable."
+  :group 'flycheck
+  :type 'string
+  :package-version '(flycheck . "0.26"))
+
 (defcustom flycheck-error-list-mode-line
   `(,(propertized-buffer-identification "%12b")
     " for buffer "
@@ -3072,7 +3084,7 @@ nil."
                      "")))
                 (`interrupted "-")
                 (`suspicious "?"))))
-    (concat " FlyC" text)))
+    (concat " " flycheck-mode-line-prefix text)))
 
 
 ;;; Error levels

test/flycheck-test.el

@@ -1745,6 +1745,15 @@ and extension, as in `file-name-base'."
            (list (flycheck-error-new-at 1 1 'info "info"))))
       (should (string= (flycheck-mode-line-status-text 'finished) " FlyC")))))
 
+(ert-deftest flycheck-mode-line/customize-lighter ()
+  :tags '(status-reporting)
+  (flycheck-ert-with-temp-buffer
+    (let ((flycheck-mode-line-prefix "FFFLLLYYYCCCHHHEEECCCKKK")
+          (flycheck-current-errors
+           (list (flycheck-error-new-at 1 1 'info "info"))))
+      (should (string= (flycheck-mode-line-status-text 'finished)
+                       " FFFLLLYYYCCCHHHEEECCCKKK")))))
+
 
 ;;; Error levels
 ;; A level for the following unit tests