youtube-dl fails to build in trace mode because python is only a runtime, not a build-time dependency

[neverpanic (Clemens Lang)]

youtube-dl uses Python during its build, but only declares a build-time dependency. This makes the build fail in trace mode, because runtime dependencies are not expected to be used during the build and thus hidden. The error is:

:info:build make: Entering directory `/opt/local/var/macports/build/_opt_dports_net_youtube-dl/youtube-dl/work/youtube-dl'
:info:build zip -q youtube-dl youtube_dl/*.py youtube_dl/*/*.py
:info:build /opt/local/bin/python2.7 devscripts/make_contributing.py README.md CONTRIBUTING.md
:info:build make: /opt/local/bin/python2.7: No such file or directory
:info:build /opt/local/bin/python2.7 devscripts/make_supportedsites.py docs/supportedsites.md
:info:build make: /opt/local/bin/python2.7: No such file or directory
:info:build make: *** [CONTRIBUTING.md] Error 1
:info:build make: *** Waiting for unfinished jobs....
:info:build make: *** [supportedsites] Error 1

and later on:

:warn:build The following existing file was hidden from the build system by trace mode:
:msg:build   /opt/local/bin/python2.7

Here's a patch:

Portfile

@@ -64 +64 @@
 default_variants    +ffmpeg
 
 variant python27 conflicts python34 python35 description {Use Python 2.7} {
-    depends_run-append  port:python27
-    configure.python    ${prefix}/bin/python2.7
+    depends_run-append      port:python27
+    depends_build-append    port:python27
+    configure.python        ${prefix}/bin/python2.7
 }
 
 variant python33 requires python34 description {Legacy compatibility variant} {}
 
 variant python34 conflicts python27 python35 description {Use Python 3.4} {
-    depends_run-append  port:python34
-    configure.python    ${prefix}/bin/python3.4
+    depends_run-append      port:python34
+    depends_build-append    port:python34
+    configure.python        ${prefix}/bin/python3.4
 }
 
 variant python35 conflicts python27 python34 description {Use Python 3.5} {
-    depends_run-append  port:python35
-    configure.python    ${prefix}/bin/python3.5
+    depends_run-append      port:python35
+    depends_build-append    port:python35
+    configure.python        ${prefix}/bin/python3.5
 }
 
 if {![variant_isset python27] && ![variant_isset python34] && ![variant_isset python35]} {

[neverpanic (Clemens Lang)]

Committed in r143292, maintainer timeout.