Fix partial tree checkouts with 'all-depends-list', 'make clean', etc, after

inclusion of the ports_env feature into that handling around r399791.

With hat:	portmgr
Reported by:	ian, lev

Mk/Scripts/functions.sh

@@ -164,7 +164,7 @@ export_ports_env() {
 
 	validate_env MAKE PORTSDIR
 
-	uses="perl5 python"
+	uses="python"
 
 	make_env="\
 		_PORTS_ENV_CHECK=1 \

Mk/Uses/perl5.mk

@@ -43,9 +43,7 @@ IGNORE=	Incorrect 'USES+=perl5:${perl5_ARGS}' perl5 takes no arguments
 USE_PERL5?=	run build
 
 # remove when 5.20 goes away.
-.if !defined(_PORTS_ENV_CHECK)
 .sinclude "${LOCALBASE}/etc/perl5_version"
-.endif
 .if defined(PERL_VERSION)
 PERL5_DEPEND=	${PERL5}
 THIS_IS_OLD_PERL=	yes

Mk/Uses/python.mk

@@ -385,10 +385,13 @@ PYTHON_MAJOR_VER=	${PYTHON_VER:R}
 PYTHON_REL=		# empty
 PYTHON_ABIVER=		# empty
 PYTHON_PORTSDIR=	${_PYTHON_RELPORTDIR}${PYTHON_SUFFIX}
+# Protect partial checkouts from Mk/Scripts/functions.sh:export_ports_env().
+.if !defined(_PORTS_ENV_CHECK) || exists(${PYTHON_PORTSDIR})
 .if !defined(PYTHON_PORTVERSION)
 PYTHON_PORTVERSION!=	${MAKE} -V PORTVERSION -C ${PYTHON_PORTSDIR}
 .endif
 _EXPORTED_VARS+=	PYTHON_PORTVERSION
+.endif
 # Create a 4 integer version string, prefixing 0 to the last token if
 # it's a single character. Only use the the first 3 tokens of
 # PORTVERSION to support pre-release versions (rc3, alpha4, etc) of