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

build/linux/pkg-config-wrapper

Index: build/linux/pkg-config-wrapper
diff --git a/build/linux/pkg-config-wrapper b/build/linux/pkg-config-wrapper
index c39e5cd3f38c3a02d5b080e8590d2d17ac30b22c..385ac29ce96cb5b27d209490efcfb44d781148b5 100755
--- a/build/linux/pkg-config-wrapper
+++ b/build/linux/pkg-config-wrapper
@@ -33,7 +33,13 @@ 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,