allow to build on recent head

The distfile is using the FreeBSD make framework
provided in sources to build, but still defines NO_MAN which is
not supported for a while to define it expects no manual pages, the
properly to define it now is to declare an empty MAN variable.

While here use options helpers from the ports framework

sysutils/env4801/Makefile

@@ -17,7 +17,6 @@ NO_WRKSUBDIR=		yes
 EXTRACT_CMD=		${CP}
 EXTRACT_BEFORE_ARGS=
 EXTRACT_AFTER_ARGS=	${WRKSRC}
-MK_MAN=			no
 
 PLIST_FILES=		sbin/env4801 etc/rc.d/env4801.sh
 
@@ -27,12 +26,8 @@ OPTIONS_DEFINE=	STATIC
 OPTIONS_DEFAULT=	STATIC
 STATIC_DESC=	Build a statically linked executable
 
-.include <bsd.port.options.mk>
-
-post-patch:
-.if ! ${PORT_OPTIONS:MSTATIC}
+post-patch-STATIC-off:
 	@${REINPLACE_CMD} -e '/-static/d' ${WRKSRC}/Makefile
-.endif
 
 do-install:
 	${INSTALL_PROGRAM} ${WRKSRC}/env4801 ${STAGEDIR}${PREFIX}/sbin

sysutils/env4801/files/patch-Makefile

@@ -0,0 +1,11 @@
+--- Makefile.orig	2021-01-12 14:27:18 UTC
++++ Makefile
+@@ -2,7 +2,7 @@ PROG=	env4801
+ 
+ WARNS?=	3
+ 
+-NO_MAN=	sorry
++MAN=
+ 
+ #CFLAGS += -fno-builtin-ffs
+