depends: disable builtin rules and suffixes.

When there is no rule to build a target in the makefile, make looks
for a builtin rule.
When -r is specified make no longer performs this lookup.

E.g. the following in an excerpt from make -d output.
Here, make looks for a rule to build 'all'.

Considering target file 'all'.
 File 'all' does not exist.
 Looking for an implicit rule for 'all'.
 Trying pattern rule with stem 'all'.
 Trying implicit prerequisite 'all.o'.
 Trying pattern rule with stem 'all'.
 Trying implicit prerequisite 'all.c'.
 Trying pattern rule with stem 'all'.
 Trying implicit prerequisite 'all.cc'.
 Trying pattern rule with stem 'all'.
 Trying implicit prerequisite 'all.C'.
 Trying pattern rule with stem 'all'.
 Trying implicit prerequisite 'all.cpp'.
 Trying pattern rule with stem 'all'.
 Trying implicit prerequisite 'all.p'.
 Trying pattern rule with stem 'all'.
 Trying implicit prerequisite 'all.f'.
...
Many more lines like this are omitted.

Because this build system does not use make builtin rules or suffixes,
there is no benefit in having builtin rules enabled.

There are 2 benefits in having builtin rules disabled.

1. Improves performance by eliminating redundant lookups.
2. Simplifies troubleshooting by reducing the output of make -d or
make -p.

Author: gtkiller

depends/Makefile

@@ -1,4 +1,6 @@
 .NOTPARALLEL :
+# Disable builtin rules and suffixes.
+MAKEFLAGS += -r
 
 # Pattern rule to print variables, e.g. make print-top_srcdir
 print-%: FORCE