don't clobber PKG_CONFIG_PATH, allow cross-compilation without a sysroot

build/linux/pkg-config-wrapper

 #!/bin/bash
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This program wraps around pkg-config to generate the correct include and
 # library paths when cross-compiling using a sysroot.
 # The assumption is that the sysroot contains the .pc files in usr/lib/pkgconfig
 # and usr/share/pkgconfig (relative to the sysroot) and that they output paths
 # relative to some parent path of the sysroot.
@@ skipping 15 matching lines @@
 if [ "$target_arch" = "x64" ]
 then
   libpath="lib64"
 else
   libpath="lib"
 fi
 
 rewrite=`dirname $0`/rewrite_dirs.py
 package=${!#}
 
-config_path=$root/usr/$libpath/pkgconfig:$root/usr/share/pkgconfig
+if [ -z "$PKG_CONFIG_PATH" ]
+then
+  config_path=$root/usr/$libpath/pkgconfig:$root/usr/share/pkgconfig
+else
+  config_path="$PKG_CONFIG_PATH"
+fi
+
 set -e
 # Some sysroots, like the Chromium OS ones, may generate paths that are not
 # relative to the sysroot. For example,
 # /path/to/chroot/build/x86-generic/usr/lib/pkgconfig/pkg.pc may have all paths
 # relative to /path/to/chroot (i.e. prefix=/build/x86-generic/usr) instead of
 # relative to /path/to/chroot/build/x86-generic (i.e prefix=/usr).
 # To support this correctly, it's necessary to extract the prefix to strip from
 # pkg-config's |prefix| variable.
 prefix=`PKG_CONFIG_PATH=$config_path pkg-config --variable=prefix "$package" | sed -e 's|/usr$||'`
 result=`PKG_CONFIG_PATH=$config_path pkg-config "$@"`
 echo "$result"| $rewrite --sysroot "$root" --strip-prefix "$prefix"