Fix pkg-config-wrapper to work with chromiumos chroots

build/linux/pkg-config-wrapper

-#!/bin/sh
+#!/bin/bash
 
 root="$1"
 if [ -z "$root" ]
 then
-  echo "usage: $0 /path/to/sysroot [pkg-config-arguments]" >&2
+  echo "usage: $0 /path/to/sysroot [pkg-config-arguments] package" >&2
   exit 1
 fi
 
 rewrite=`dirname $0`/rewrite_dirs.py
+package=${!#}
 
 shift
 config_path=$root/usr/lib/pkgconfig:$root/usr/share/pkgconfig
 set -e
-result=`PKG_CONFIG_PATH=$config_path pkg-config --define-variable=prefix=/usr "$@"`
-echo "$result"| $rewrite $root
+# Some sysroots, like the Chromium OS ones, may generate paths that are not
+# relative to the sysroot. For example,
+# /path/to/chroot/build/x86-generic/usr/lib/pkgconfig/pkg.pc may have all paths
+# relative to /path/to/chroot (i.e. prefix=/build/x86-generic/usr) instead of
+# relative to /path/to/chroot/build/x86-generic (i.e prefix=/usr).
+# To support this correctly, it's necessary to extract the prefix to strip from
+# pkg-config's |prefix| variable.
+prefix=`PKG_CONFIG_PATH=$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"