Fix check for signs support in Neovim (#402)

Fixes #322

Author: rkiyanchuk

autoload/lsp/ui/vim/signs.vim

@@ -1,7 +1,7 @@
 " TODO: handle !has('signs')
 " TODO: handle signs clearing when server exits
 " https://github.com/vim/vim/pull/3652
-let s:supports_signs = has('signs') && has('patch-8.1.0772') && exists('*sign_define')
+let s:supports_signs = exists('*sign_define') && (has('nvim') || has('patch-8.1.0772'))
 let s:enabled = 0
 let s:signs = {} " { server_name: { path: {} } }
 let s:severity_sign_names_mapping = {

plugin/lsp.vim

@@ -8,7 +8,7 @@ let g:lsp_async_completion = get(g:, 'lsp_async_completion', 0)
 let g:lsp_log_file = get(g:, 'lsp_log_file', '')
 let g:lsp_log_verbose = get(g:, 'lsp_log_verbose', 1)
 let g:lsp_debug_servers = get(g:, 'lsp_debug_servers', [])
-let g:lsp_signs_enabled = get(g:, 'lsp_signs_enabled', has('patch-8.1.0772') && exists('*sign_define'))
+let g:lsp_signs_enabled = get(g:, 'lsp_signs_enabled', exists('*sign_define') && (has('nvim') || has('patch-8.1.0772')))
 let g:lsp_virtual_text_enabled = get(g:, 'lsp_virtual_text_enabled', exists('*nvim_buf_set_virtual_text'))
 let g:lsp_highlights_enabled = get(g:, 'lsp_highlights_enabled', exists('*nvim_buf_add_highlight'))
 let g:lsp_textprop_enabled = get(g:, 'lsp_textprop_enabled', exists('*prop_add') && !g:lsp_highlights_enabled)