The RPATH support added a @{libdir} token into
<package>_WRAPPER_EXTRA_LDFLAGS. However, these flags are also
substituted into the pkg-config data files, and they don't understand
the @{foo} notation. So convert @{libdir} into ${libdir}, which
pkg-config *does* understand.
Thanks to Christoph Junghans (@junghans) for notifying us of the issue.
Fixes#406.