vim-plug update, guard for YCM

vimrc

@@ -93,6 +93,20 @@ inoremap   <Up>        <C-o>gk
 nnoremap   <Down>      gj
 nnoremap   <Up>        gk
 
+function! YCMInstallCmd()
+	let l:ycm_cmd = 'python2 ./install.py'
+	if executable('c++')
+		let l:ycm_cmd .= ' --clang-completer'
+	endif
+	if executable('go')
+		let l:ycm_cmd .= ' --gocode-completer'
+	endif
+	if executable('npm')
+		let l:ycm_cmd .= ' --tern-completer'
+	endif
+	return l:ycm_cmd
+endfunction
+
 "execute pathogen#infect()
 call plug#begin('~/.vim/plugged')
 Plug 'murukeshm/vim-manpager'
@@ -112,9 +126,11 @@ Plug 'gabrielelana/vim-markdown', {'for': 'markdown'}
 Plug 'majutsushi/tagbar', {'for': ['cpp', 'c', 'go', 'sh', 'js']} 
 Plug 'fatih/vim-go', {'for': 'go'}
 
-" YCM command lifted from vim-plug readme
-Plug 'Valloric/YouCompleteMe', { 'do': 'python2 ./install.py --clang-completer --gocode-completer --tern-completer', 'for': ['cpp', 'c', 'go', 'sh', 'js', 'vim'] }
-autocmd! User YouCompleteMe if !has('vim_starting') | call youcompleteme#Enable() | endif
+if executable('cmake')
+	" YCM command lifted from vim-plug readme
+	Plug 'Valloric/YouCompleteMe', { 'do': YCMInstallCmd(), 'for': ['cpp', 'c', 'go', 'sh', 'js', 'vim'] }
+	autocmd! User YouCompleteMe if !has('vim_starting') | call youcompleteme#Enable() | endif
+endif
 
 Plug 'godlygeek/tabular'