Check .vimrc '+shellslash' before setting it

From :help 'shellslash':

> 'shellslash' only works when a backslash can be used as a path
> separator.  To test if this is so use:
>     if exists('+shellslash')

1 files changed, 3 insertions, 1 deletions

diff --git a/vim/vimrc b/vim/vimrc
index b64e4290..dd8d9232 100644
--- a/vim/vimrc
+++ b/vim/vimrc
@@ -208,7 +208,9 @@ set report=0
 
 " Always use forward slashes, I very seldom need to use Vim on Windows for
 " more than scratch space anyway
-set shellslash
+if exists('+shellslash')
+  set shellslash
+fi
 
 " Don't show line numbers by default, but \n toggles them
 set nonumber