[tmux] fix immediate exit on Linux

The fix in commit 6bd38b874978e96dc2632abf1c98460e908fa6a6 causes tmux
to exit immediately when running on Linux, since the
reattach-to-user-namespace is specific to MacOS and the binary is
therefore not available on Linux.

More details at:

    http://www.economyofeffort.com/2013/07/29/reattach-to-user-namespace-the-fix-for-your-tmux-in-os-x-woes/

tmux-macos.conf

@@ -0,0 +1,5 @@
+# Fix for clipboard in vim running inside tmux:
+#   See http://stackoverflow.com/a/40154047
+#   See https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard
+set -g default-command "reattach-to-user-namespace -l $SHELL"
+

tmux.conf

@@ -24,7 +24,6 @@ bind -r C-j     resize-pane -D 5
 bind -r C-k     resize-pane -U 5
 bind -r C-l     resize-pane -R 5
 
-
 #
 # General
 #
@@ -61,3 +60,7 @@ setw -g window-status-current-bg    default
 set -g pane-border-fg               colour0     # Base02
 set -g pane-active-border-fg        colour9     # Orange
 
+# Apply platform specific configurations:
+#   See http://www.economyofeffort.com/2013/07/29/reattach-to-user-namespace-the-fix-for-your-tmux-in-os-x-woes/
+if-shell 'test "$(uname)" = "Darwin"' 'source ~/.tmux-macos.conf'
+