linux (sysroot): force prefix=/usr in pkg-config

This lets us use portage-based cross-compiled sysroots, that force the prefix to be an absolute path within the portage chroot.
In particular that lets us build the ARM sysroot using the Chrome OS build system.
Ubuntu-based sysroots already have prefix=/usr, so this is a no-op in that case.

Review URL: http://codereview.chromium.org/669139

git-svn-id: http://src.chromium.org/svn/trunk/src/build@40712 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

linux/pkg-config-wrapper

@@ -11,4 +11,4 @@ rewrite=`dirname $0`/rewrite_dirs.py
 
 shift
 config_path=$root/usr/lib/pkgconfig:$root/usr/share/pkgconfig
-PKG_CONFIG_PATH=$config_path pkg-config "$@" | $rewrite $root
+PKG_CONFIG_PATH=$config_path pkg-config --define-variable=prefix=/usr "$@" | $rewrite $root