diff --git a/digest.cabal b/digest.cabal
index b958879..d9a2498 100644
--- a/digest.cabal
+++ b/digest.cabal
@@ -25,6 +25,11 @@ extra-source-files:
   testing/trivial.hs
   CHANGELOG.md
 
+flag pkg-config
+  default:     True
+  manual:      True
+  description: Use @pkg-config(1)@ to locate @zlib@ library.
+
 source-repository head
   type: git
   location: https://github.com/TeofilC/digest
@@ -39,7 +44,7 @@ library
     , bytestring >= 0.9 && < 0.12
   includes:        zlib.h
   ghc-options:     -Wall
-  if !os(windows)
+  if flag(pkg-config) && !os(windows)
     pkgconfig-depends: zlib
   else
     build-depends: zlib