Cleaner fix for printing make rules without setting an apparently unr…

…elated variable.

build/make/Makefile.in

@@ -9,14 +9,15 @@
 # obscure the substance of the actual rules, this file can be debugged by
 # running:
 #
-#     $ make -f build/make/Makefile -n DEBUG_RULES=1 SAGE_PKGCONFIG=1
+#     $ make -f build/make/Makefile -n DEBUG_RULES=1
 #
 # This will not actually run any rules (the -n flag) but will print all the
 # rules generated from the templates.
 
 # Always use bash for make rules
 SHELL = @SHELL@
 
+ifndef DEBUG_RULES
 # Check a variable that is only set in build/make/install, but not in sage-env, for example
 ifndef SAGE_PKGCONFIG
 # Set by build/bin/sage-sdist, which invokes the Makefile directly in
@@ -25,6 +26,7 @@ ifndef SAGE_SPKG_COPY_UPSTREAM
 $(error This Makefile needs to be invoked by build/make/install)
 endif
 endif
+endif
 
 # Directory to keep track of which packages are installed - relative to installation prefix
 SPKG_INST_RELDIR = var/lib/sage/installed