RFR: JDK-8068726: Tab completion of targets fails when current dir is the output dir

[Erik Joelsson]

Hello,

Please review this minor fix. Tab completion stopped working when 
running make from the output dir (which I often do). I found that this 
fixed the problem:

diff -r ef5c7075496d Makefile
--- a/Makefile
+++ b/Makefile
@@ -54,8 +54,11 @@
    # Duplication of global targets, needed before ParseConfAndSpec in 
case we have
    # no configurations.
    help:
-  # If CONF is not set, look for all available configurations
-  CONF?=
+  # If both CONF and SPEC are unset, look for all available 
configurations by
+  # setting CONF to the empty string.
+  ifeq ($(SPEC), )
+    CONF?=
+  endif
  endif

Bug: https://bugs.openjdk.java.net/browse/JDK-8068726

/Erik