Mk/Uses: Add elfctl

Add USES=elfctl to change an ELF binary's feature control note.

Example:

USES=           elfctl
ELF_FEATURES=   +noaslr,wxneeded:foo

Reported by:	emaste
Approved by:	brooks, tcberner (portmgr)
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D34125

Mk/Uses/elfctl.mk

@@ -0,0 +1,27 @@
+# Change an ELF binary's feature control note
+#
+# Feature:	elfctl
+# Usage:	USES=elfctl
+# Valid ARGS:	none
+#
+# Variables
+#
+#   ELF_FEATURES=	featurelist:path/to/file1 \
+#			featurelist:path/to/file1 \
+			featurelist:path/to/file2
+#
+# The file paths listed in ELF_FEATURES are relative to ${BUILD_WRKSRC}.
+#
+
+.if !defined(_INCLUDE_USES_ELFCTL_MK)
+_INCLUDE_USES_ELFCTL_MK=	yes
+
+.  if ! empty(ELF_FEATURES)
+_USES_build+=	720:elfctl-post-build
+elfctl-post-build:
+.    for feat in ${ELF_FEATURES}
+	${ELFCTL} -i -e ${feat:C/:.*//} ${BUILD_WRKSRC}/${feat:C/.*://}
+.    endfor
+.  endif
+
+.endif

Mk/bsd.commands.mk

@@ -34,6 +34,7 @@ DIALOG4PORTS?=		${LOCALBASE}/bin/dialog4ports
 DIFF?=			/usr/bin/diff
 DIRNAME?=		/usr/bin/dirname
 EGREP?=			/usr/bin/egrep
+ELFCTL?=		/usr/bin/elfctl
 EXPR?=			/bin/expr
 FALSE?=			false	# Shell builtin
 FILE?=			/usr/bin/file