Set bindir in pkgconfig file and use it.
diff --git a/libzip.pc.in b/libzip.pc.in
index 53abc47..7be7f3e 100644
--- a/libzip.pc.in
+++ b/libzip.pc.in
@@ -1,9 +1,10 @@
 prefix=@prefix@
 exec_prefix=@exec_prefix@
+bindir=@bindir@
 libdir=@libdir@
 includedir=@includedir@
 
-zipcmp=@prefix@/bin/zipcmp
+zipcmp=@bindir@/zipcmp
 
 Name: libzip
 Description: library for handling zip archives