selinux: fix Makefile for versions of make < v4.3

As noted in the comments of this commit, the current SELinux Makefile
requires features found in make v4.3 or later, which is problematic
as the Linux Kernel currently only requires make v3.82.  This patch
fixes the SELinux Makefile so that it works properly on these older
versions of make, and adds a couple of comments to the Makefile about
how it can be improved once make v4.3 is required by the kernel.

Fixes: 6f933aa7 ("selinux: more Makefile tweaks")
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/Makefile

@@ -3,6 +3,11 @@
 # Makefile for building the SELinux module as part of the kernel tree.
 #
 
+# NOTE: There are a number of improvements that can be made to this Makefile
+# once the kernel requires make v4.3 or greater; the most important feature
+# lacking in older versions of make is support for grouped targets.  These
+# improvements are noted inline in the Makefile below ...
+
 obj-$(CONFIG_SECURITY_SELINUX) := selinux.o
 
 ccflags-y := -I$(srctree)/security/selinux -I$(srctree)/security/selinux/include
@@ -19,11 +24,17 @@ selinux-$(CONFIG_IMA) += ima.o
 
 genhdrs := flask.h av_permissions.h
 
-$(addprefix $(obj)/,$(selinux-y)): $(addprefix $(obj)/,$(genhdrs))
+# see the note above, replace the dependency rule with the one below:
+#  $(addprefix $(obj)/,$(selinux-y)): $(addprefix $(obj)/,$(genhdrs))
+$(addprefix $(obj)/,$(selinux-y)): $(obj)/flask.h
 
 quiet_cmd_genhdrs = GEN     $(addprefix $(obj)/,$(genhdrs))
       cmd_genhdrs = $< $(addprefix $(obj)/,$(genhdrs))
 
-targets += $(genhdrs)
-$(addprefix $(obj)/,$(genhdrs)) &: scripts/selinux/genheaders/genheaders FORCE
+# see the note above, replace the $targets and 'flask.h' rule with the lines
+# below:
+#  targets += $(genhdrs)
+#  $(addprefix $(obj)/,$(genhdrs)) &: scripts/selinux/...
+targets += flask.h
+$(obj)/flask.h: scripts/selinux/genheaders/genheaders FORCE
 	$(call if_changed,genhdrs)