tests: less strict double-colon spy.test.

[Ralf Wildenhues]

I wasn't completely sure about the documentation in spy.test, but a
recheck shows that FreeBSD, NetBSD, and OpenBSD make do not invoke both
rules if the target is out of date wrt. both prerequisites.  So let's
make the spy less strict again so it has a chance to pass on all known
systems at least.  I'm leaving the code in, commented, so there's no
way confusing the explanation another time.

For maint again.

Cheers,
Ralf

    tests: less strict double-colon spy.test again.
    
    * tests/spy.test: We know BSD make doesn't invoke more than one
    double-colon rule, so no need to expose that failure.

diff --git a/tests/spy.test b/tests/spy.test
index ab68a61..5f1a87b 100755
--- a/tests/spy.test
+++ b/tests/spy.test
@@ -94,11 +94,15 @@ $sleep
 touch c
 $MAKE
 test "`cat a`" = rule2
-: > a
-$sleep
-touch b c
-$MAKE
-grep rule1 a
-grep rule2 a
+
+# Unfortunately, the following is not portable to FreeBSD/NetBSD/OpenBSD
+# make, see explanation above.
+
+#: > a
+#$sleep
+#touch b c
+#$MAKE
+#grep rule1 a
+#grep rule2 a
 
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