vimrc: Disable gutentags when ctags unavailable

.vimrc

@@ -113,6 +113,9 @@ command! -bang -range=% -complete=file -nargs=* W <line1>,<line2>write<bang> <ar
 command! -bang Q quit<bang>
 
 " Gutentags configuration
+if !executable('ctags')
+    let g:gutentags_enabled = 0
+else
     let g:gutentags_add_default_project_roots = 0
     let g:gutentags_project_root = ['.git']
 
@@ -178,6 +181,7 @@ let g:gutentags_ctags_exclude = [
           \ '*.pdf', '*.doc', '*.docx', '*.ppt', '*.pptx',
           \ ]
 
+endif
 
 " fugitive.vim looks for tags in .git
 " set tags +=~/tags " Recursively move upwards in tree, searching in subfolders for tags file