[PATCH] selftests: more general make nesting support

[Greg Thelen <gthelen@google.com>]

selftests can be built from the toplevel kernel makefile (e.g. make
kselftest-all) or directly (make -C tools/testing/selftests all).

The toplevel kernel makefile explicitly disables implicit rules with
"MAKEFLAGS += -rR", which is passed to tools/testing/selftests.  Some
selftest makefiles require implicit make rules, which is why
commit 67d8712dcc70 ("selftests: Fix build failures when invoked from
kselftest target") reenables implicit rules by clearing MAKEFLAGS if
MAKELEVEL=1.

So far so good.  However, if the toplevel makefile is called from an
outer makefile then MAKELEVEL will be elevated, which breaks the
MAKELEVEL equality test.
Example wrapped makefile error:
  $ cat ~/Makefile
  all:
  	$(MAKE) defconfig
  	$(MAKE) kselftest-all
  $ make -sf ~/Makefile
    futex_wait_timeout.c /src/tools/testing/selftests/kselftest_harness.h   /src/tools/testing/selftests/kselftest.h ../include/futextest.h ../include/atomic.h ../include/logging.h -lpthread -lrt -o /src/tools/testing/selftests/futex/functional/futex_wait_timeout
  make[4]: futex_wait_timeout.c: Command not found

Rather than checking $(MAKELEVEL), check for $(LINK.c), which is a more
direct side effect of "make -R".  This enables arbitrary makefile
nesting.

Signed-off-by: Greg Thelen <gthelen@google.com>
---
 tools/testing/selftests/Makefile | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 1195bd85af38..289a2e4b3f6f 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -84,10 +84,10 @@ endif
 # of the targets gets built.
 FORCE_TARGETS ?=
 
-# Clear LDFLAGS and MAKEFLAGS if called from main
-# Makefile to avoid test build failures when test
-# Makefile doesn't have explicit build rules.
-ifeq (1,$(MAKELEVEL))
+# Clear LDFLAGS and MAKEFLAGS when implicit rules are missing.  This provides
+# implicit rules to sub-test Makefiles which avoids build failures in test
+# Makefile that don't have explicit build rules.
+ifeq (,$(LINK.c))
 override LDFLAGS =
 override MAKEFLAGS =
 endif
-- 
2.28.0.rc0.142.g3c755180ce-goog