[bug #20501] "MAKEFLAGS += -rR" doesn't turn off default suffix rules, variables

[Paul D. Smith]

Follow-up Comment #9, bug #20501 (project make):

It's not completely true that -p shows "the rules as if they were there". 
Recall that there are two types of implicit rules in GNU make: suffix rules
and pattern rules.

If you use MAKEFLAGS+=-r then the _pattern rules_ will not be present in the
-p output.

However, MAKEFLAGS+=-r is not the same as "make -r" because the latter ALSO
prevents suffix rules from being defined, while the former does not.  The
former only applies to pattern rules.

That's why in the makefile you need BOTH the .SUFFIXES: special target AND the
MAKEFLAGS+=-r.  Having the .SUFFIXES: special target set to empty ensures that
no suffix rules are searched but it does NOT remove all the suffix rules from
the database.  So they still show up with -p, but they don't have any impact
on the algorithms make uses to build targets.

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