configure: Don't run "export $e" for an empty string

This fixes the side effect of printing the whole environment,
if no --env parameter was provided.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -2543,7 +2543,7 @@ for opt do
     esac
 done
 
-for e in "$env"; do
+for e in $env; do
     eval "export $e"
 done