#21 packaging pkgconfig - use @libdir@ subst

[Guido U. Draheim]

The current pkgconfig template has a hardcoded
$prefix/lib which is wrong when compiling Quickfix
for a 64-bit Linux target. Instead use the autoconf
$libdir variable directly.

The patch does it as well for the other paths that
configure.in provides anyway.

Tested at https://build.opensuse.org/package/show?package=quickfix&project=home%3Aguidod%3Afixfast

[Guido U. Draheim]