Use the same ${pkgsysconfdir} everywhere

The pkg-config file wasn't using the one defined by the configure
script and could lead to a mismatch if the package name changes.

varnishapi.pc.in

@@ -4,7 +4,7 @@ bindir=@bindir@
 sbindir=@sbindir@
 libdir=@libdir@
 sysconfdir=@sysconfdir@
-pkgsysconfdir=${sysconfdir}/@PACKAGE@
+pkgsysconfdir=@pkgsysconfdir@
 includedir=@includedir@
 pkgincludedir=${includedir}/@PACKAGE@
 datarootdir=@datarootdir@