Do not clobber PKG_CONFIG_PATH during configure.

--- HandBrake-0.10.5/gtk/module.rules.orig	2016-02-11 14:14:05.000000000 -0600
+++ HandBrake-0.10.5/gtk/module.rules	2016-10-29 22:27:50.550960848 -0500
@@ -15,7 +15,7 @@
 	set -e; cd $(GTK.src/); NOCONFIGURE=1 ./autogen.sh
 	set -e; cd $(GTK.build/); $(call fn.ABSOLUTE,$(GTK.src/))configure \
 		$(GTK.CONFIGURE.extra) \
-		PKG_CONFIG_PATH=$(BUILD/)contrib/lib/pkgconfig \
+		PKG_CONFIG_PATH=$(BUILD/)contrib/lib/pkgconfig$(if $(PKG_CONFIG_PATH),:)$(PKG_CONFIG_PATH) \
 	    CFLAGS="$(call fn.ARGS,GTK.GCC,.g .O *D ?extra)" \
 	    LDFLAGS="$(call fn.ARGS,GTK.GCC,?strip .g .O ?extra.exe)" \
 	    --prefix=$(PREFIX) \
--- Handbrake-0.10.5-0.77d09e9-checkout/make/include/contrib.defs	1969-12-31 18:00:00.000000000 -0600
+++ Handbrake-0.10.5-0.77d09e9-checkout/make/include/contrib.defs	2016-11-01 13:11:43.826144311 -0500
@@ -119,7 +119,7 @@
     endif
     $(1).CONFIGURE.env.CPPFLAGS = CPPFLAGS="-I$$(call fn.ABSOLUTE,$(CONTRIB.build/))include $$(call fn.ARGS,$(1).GCC,*archs *sysroot *minver ?extra *D)"
     $(1).CONFIGURE.env.LDFLAGS  = LDFLAGS="-L$$(call fn.ABSOLUTE,$(CONTRIB.build/))lib $$(call fn.ARGS,$(1).GCC,*archs *sysroot *minver ?extra.exe *D)"
-    $(1).CONFIGURE.env.PKG_CONFIG_PATH  = PKG_CONFIG_PATH="$$(call fn.ABSOLUTE,$$(CONTRIB.build/))lib/pkgconfig"
+    $(1).CONFIGURE.env.PKG_CONFIG_PATH  = PKG_CONFIG_PATH="$$(call fn.ABSOLUTE,$$(CONTRIB.build/))lib/pkgconfig$(if $(PKG_CONFIG_PATH),:)$(PKG_CONFIG_PATH)"
 
     $(1).CONFIGURE.env.args = !CC !CFLAGS !CXX !CXXFLAGS !CPPFLAGS !LD !LDFLAGS !PKG_CONFIG_PATH !LOCAL_PATH !CROSS
     $(1).CONFIGURE.env = $$(call fn.ARGS,$(1).CONFIGURE.env,$$($(1).CONFIGURE.env.args))
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<div class='path'>path: <a href='/guix/log/?id=cc4906d13b3749c4caff6ec8034bdaf2c7ff7da2'>root</a>/<a href='/guix/log/tests?id=cc4906d13b3749c4caff6ec8034bdaf2c7ff7da2'>tests</a>/<a href='/guix/log/tests/swh.scm?id=cc4906d13b3749c4caff6ec8034bdaf2c7ff7da2'>swh.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/swh.scm?id=cc4906d13b3749c4caff6ec8034bdaf2c7ff7da2&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-01-21 11:22:08 +0100'>2021-01-21</span></td><td><a href='/guix/commit/tests/swh.scm?id=5225732b9b54ffa92b9d199246b87dae666a3f77'>swh: Test proper handling of null visit snapshot URL.</a><span class='msg-avail'>...<span class='msg-tooltip'>* tests/swh.scm (%origin): Change "visits_url" to "origin_visits_url".
(%visits): New variable.
("origin-visit, no snapshots"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-12-21 17:47:35 +0100'>2020-12-21</span></td><td><a href='/guix/commit/tests/swh.scm?id=9608f4003dedd8dfe99327c15668ca1a43ebd93b'>tests: Fix malformed JSON.</a><span class='msg-avail'>...<span class='msg-tooltip'>Guile-JSON 4.3.2 would parse in spite of these typos, but 4.4.1 is
stricter.

* tests/swh.scm (%directory-entries): Add missing comma.
* tests/cve-sample.json: Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-02-27 23:29:30 +0100'>2020-02-27</span></td><td><a href='/guix/commit/tests/swh.scm?id=749b9582061f7d929be83a194040b63b41afec81'>swh: Adjust 'origin' test.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a followup to 3d2f29382de2d0ee852745cc002dfe2b5d22e1c2.

* tests/swh.scm (%origin): Remove "id".
("lookup-origin"): Remove use of 'origin-id'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-02 15:25:01 +0200'>2019-09-02</span></td><td><a href='/guix/commit/tests/swh.scm?id=ba1c1853a79a5930ca7db7a6b368859f805df98d'>swh: Add hooks for rate limiting handling.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/swh.scm (%allow-request?, %save-rate-limit-reset-time)
(%general-rate-limit-reset-time): New variables.
(request-rate-limit-reached?, update-rate-limit-reset-time!): New
procedures.
(call): Call '%allow-request?'.  Change 'swh-error' protocol to pass
METHOD in addition to URL.
* tests/swh.scm ("rate limit reached")
("%allow-request? and request-rate-limit-reached?"): New tests.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-02 15:25:01 +0200'>2019-09-02</span></td><td><a href='/guix/commit/tests/swh.scm?id=9323ab550f3bcb75fcaefbb20847595974702d5b'>tests: 'with-http-server' accepts multiple responses.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/tests/http.scm (call-with-http-server): Replace 'code' and 'data'
parameters with 'responses+data'.  Compute RESPONSES as a function of
that.  Remove #:headers parameter.
[http-write]: Quit only when RESPONSES is empty.
[server-body]: Get the response and data from RESPONSES, and set it to
point to the rest.
(with-http-server): Adjust accordingly.
* tests/derivations.scm ("'download' built-in builder")
("'download' built-in builder, invalid hash")
("'download' built-in builder, not found")
("'download' built-in builder, check mode"): Adjust to new
'with-http-server' interface.
* tests/lint.scm ("home-page: 200")
("home-page: 200 but short length")
("home-page: 404", "home-page: 301, invalid"):
("home-page: 301 -&gt; 200", "home-page: 301 -&gt; 404")
("source: 200", "source: 200 but short length")
("source: 404", "source: 404 and 200")
("source: 301 -&gt; 200", "source: 301 -&gt; 404"):
("github-url", github-url): Likewise.
* tests/swh.scm (with-json-result)
("lookup-origin, not found"): Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-07-22 11:53:39 +0200'>2019-07-22</span></td><td><a href='/guix/commit/tests/swh.scm?id=96f1cbeff84819f9886d15763b4c477cdecd7784'>swh: Add basic tests.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/swh.scm (%swh-base-url): Turn into a parameter and export it.
* tests/swh.scm: New file.
* Makefile.am (SCM_TESTS): Add it.
</span></span></td><td>Ludovic Courtès</td></tr>