Add a USES=python:env, that will not add a dependency on Python.

It has the same effect than setting PYTHON_NO_DEPENDS, but in a more
compact way.

Sponsored by:	Absolight

Mk/Uses/python.mk

@@ -28,6 +28,11 @@
 #		it to RUN_DEPENDS.
 # test		Indicates that Python is needed at test time and adds
 # 		it to TEST_DEPENDS.
+# env		Indicates that the port does not require a dependency on Python
+#		itself but needs the environment set up. This is mainly used
+#		when depending on flavored python ports, or when a correct
+#		PYTHON_CMD is required.  It has the same effect than setting
+#		PYTHON_NO_DEPENDS.
 #
 # If build, run and test are omitted, Python will be added as BUILD_DEPENDS,
 # RUN_DEPENDS and TEST_DEPENDS. PYTHON_NO_DEPENDS can be set to not add any
@@ -282,6 +287,11 @@ _PYTHON_ARGS:=		${_PYTHON_ARGS:Nrun}
 _PYTHON_TEST_DEP=	yes
 _PYTHON_ARGS:=		${_PYTHON_ARGS:Ntest}
 .endif
+.if ${_PYTHON_ARGS:Menv}
+PYTHON_NO_DEPENDS=	yes
+_PYTHON_ARGS:=		${_PYTHON_ARGS:Nenv}
+.endif
+.endif
 
 # The port does not specify a build, run or test dependency, assume all are
 # required.

biology/bcftools/Makefile

@@ -20,9 +20,8 @@ CONFLICTS_BUILD=openssl
 USE_GITHUB=	yes
 GH_ACCOUNT=	samtools
 
-USES=		autoreconf gmake perl5 python shebangfix
+USES=		autoreconf gmake perl5 python:env shebangfix
 USE_PERL5=	test
-PYTHON_NO_DEPENDS=	yes
 GNU_CONFIGURE=	yes
 SUB_FILES=	pkg-message
 SHEBANG_FILES=	misc/* test/test.pl

biology/samtools/Makefile

@@ -17,9 +17,8 @@ TEST_DEPENDS=	bash:shells/bash
 
 USE_GITHUB=	yes
 
-USES=		autoreconf gmake ncurses perl5 python shebangfix
+USES=		autoreconf gmake ncurses perl5 python:env shebangfix
 USE_PERL5=	test
-PYTHON_NO_DEPENDS=	yes
 GNU_CONFIGURE=	yes
 SUB_FILES=	pkg-message
 SHEBANG_FILES=	misc/*.pl misc/*.py misc/plot-bamstats test/test.pl

security/metasploit/Makefile

@@ -86,9 +86,8 @@ NO_ARCH=	yes
 NO_ARCH_IGNORE=	template_x64_bsd.bin template_x86_bsd.bin CVE-2013-2171.bin
 STRIP=
 
-USES=		python shebangfix
+USES=		python:env shebangfix
 USE_RUBY=	yes
-PYTHON_NO_DEPENDS=	yes
 SHEBANG_FILES=	data/exploits/CVE-2017-7494/build.sh \
 		data/exploits/CVE-2017-7494/install-deps.sh \
 		external/source/shellcode/windows/build.sh \