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))
'committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=ea4c1e0b0e83af86decd40ee264a3aabd08abd98'>root</a>/<a href='/guix/log/HACKING?id=ea4c1e0b0e83af86decd40ee264a3aabd08abd98'>HACKING</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/HACKING?id=ea4c1e0b0e83af86decd40ee264a3aabd08abd98&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-01-09 23:33:04 +0100'>2020-01-09</span></td><td><a href='/guix/commit/HACKING?id=2d315cd428484537e763d7f7c676dc5fff7995f6'>doc: Move "Commit Access" section from 'HACKING' to the manual.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-01-09 23:33:04 +0100'>2020-01-09</span></td><td><a href='/guix/commit/HACKING?id=a7bde77d247ed0765f5378779d67b4668c5d7648'>doc: Add "Tracking Bugs and Patches" section.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-07-13 14:25:19 -0400'>2019-07-13</span></td><td><a href='/guix/commit/HACKING?id=06e1ecbf17fee7fe513ad2808d8175fb3565ae3e'>doc: Update keyserver URL.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2019-02-11 23:23:27 +0100'>2019-02-11</span></td><td><a href='/guix/commit/HACKING?id=6701edbfad62ba9572ffd35497df3db27b208ba5'>HACKING: Update key server.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-07-22 17:00:47 +0200'>2018-07-22</span></td><td><a href='/guix/commit/HACKING?id=5285192150c83db12136af6a123e03bb9b9dbe5f'>doc: Fix typo (missing period)</a><span class='msg-avail'>...</span></td><td>Pierre Neidhardt</td></tr>
<tr><td><span title='2017-08-19 13:42:52 +0530'>2017-08-19</span></td><td><a href='/guix/commit/HACKING?id=acabbd597678e1f0be995297bb009b6dd38ae795'>doc: Mention how to use emacs-debbugs.</a><span class='msg-avail'>...</span></td><td>Arun Isaac</td></tr>