Follow-up r320061: Need to respect make.conf/env LIBDIR overrides.

This fixes the lib32 build from creating all stale .depend files.

X-MFC-With:	320061
MFC after:	1 week
Sponsored by:	Dell EMC Isilon

share/mk/bsd.own.mk

@@ -152,6 +152,11 @@ DTBOWN?=	root
 DTBGRP?=	wheel
 DTBMODE?=	444
 
+# Use make.conf / environment LIBDIR as default if set...
+.if !empty(_PREMK_LIBDIR)
+LIBDIR_BASE?=	${_PREMK_LIBDIR}
+.endif
+# otherwise use our expected default value.
 LIBDIR_BASE?=	/usr/lib
 LIBDIR?=	${LIBDIR_BASE}
 LIBCOMPATDIR?=	/usr/lib/compat

share/mk/local.sys.mk

@@ -42,6 +42,10 @@ MAKE_PRINT_VAR_ON_ERROR += .MAKE.MAKEFILES .PATH
 OBJTOP?= ${.OBJDIR:S,${.CURDIR},,}${SRCTOP}
 .endif
 
+.if !empty(LIBDIR)
+_PREMK_LIBDIR:=	${LIBDIR}
+.endif
+
 .include "src.sys.mk"
 
 .if ${.MAKE.MODE:Mmeta*} != ""