patch 8.0.1554: custom plugins loaded with --clean

Problem:    Custom plugins loaded with --clean.
Solution:   Do not include the home directory in 'runtimepath'.

src/os_mac.h

@@ -215,6 +215,9 @@
 #ifndef DFLT_RUNTIMEPATH
 # define DFLT_RUNTIMEPATH	"~/.vim,$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after,~/.vim/after"
 #endif
+#ifndef CLEAN_RUNTIMEPATH
+# define CLEAN_RUNTIMEPATH	"$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after"
+#endif
 
 /*
  * Macintosh has plenty of memory, use large buffers