use poor man's shellescape in case %:S unavailable

Konfekt · da-x · commit 255a81f091a0 · 2020-03-06T09:30:39.000+02:00
diff --git a/compiler/rustc.vim b/compiler/rustc.vim
@@ -24,7 +24,7 @@ else
     if has('patch-7.4.191')
       CompilerSet makeprg=rustc\ \%:S
     else
-      CompilerSet makeprg=rustc\ \%
+      CompilerSet makeprg=rustc\ \"%\"
     endif
 endif
 
