Merged in changes between PRE11_ROOT and LLVM_PRE111 (i.e. the beginning of

the pre-release 1.1 branch and pre-release 1.1.1).
Made the USE_SPEC option work.
Silenced unnecessary error output from the cmp command when checking for
updates to lex/yacc generated files.  This fixes a problem where we get error
messages the first time the file is generated.
Fixed the distclean option.  It is now in the Makefile (i.e. only runs in the
top level source directory), removes more files, and plays nicely with
external project Makefiles.

llvm-svn: 7780