Fix recursion:

1. Make setting OPTIONAL_DIRS use immediate assignment
2. Include Makefile.config before ifeq tests and then Makefile.rules later
   instead of Makefile.common up front. This ensures that the variable
   values are set before the ifeq statements in Makefile.rules are
   evaluated.
With this, recursion into projects/examples is corrected.

llvm-svn: 27519