npth: fix library path on configure

Updating config line to system
  "./configure --prefix=#{CREW_PREFIX} --libdir=#{CREW_LIB_PREFIX}"

[Ticket: #1610]

packages/npth.rb

@@ -21,7 +21,7 @@ class Npth < Package
   })
 
   def self.build
-    system "./configure --prefix=/usr/local"
+	  system "./configure --prefix=#{CREW_PREFIX} --libdir=#{CREW_LIB_PREFIX}"
     system "make"
   end