add: support config option for checkmake linter (#4351)

`checkmake` by default checks config file "in the same folder it's
executed in" unless `--config` option is set.

This commit allows setting the `--config` option with an option
variable (with documentation updated).

doc/ale-make.txt

@@ -0,0 +1,18 @@
+===============================================================================
+ALE Make Integration                                           *ale-make-options*
+
+
+===============================================================================
+checkmake                                                    *ale-make-checkmake*
+
+g:ale_make_checkmake_config                         *g:ale_make_checkmake_config*
+                                                    *b:ale_make_checkmake_config*
+  Type: |String|
+  Default: `''`
+
+  This variable can be used to set the `--config` option of checkmake command.
+  if the value is empty, the checkmake command will not be invoked with the
+  option.
+
+===============================================================================
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale.txt

@@ -3037,6 +3037,8 @@ documented in additional help files.
     luafmt................................|ale-lua-luafmt|
     selene................................|ale-lua-selene|
     stylua................................|ale-lua-stylua|
+  make....................................|ale-make-options|
+    checkmake.............................|ale-make-checkmake|
   markdown................................|ale-markdown-options|
     cspell................................|ale-markdown-cspell|
     dprint................................|ale-markdown-dprint|