Make "unbuildable on windows" explicit

hpath.cabal

@@ -17,6 +17,9 @@ extra-source-files:  README.md
                      doctests-posix.hs
 
 library
+  if os(windows)
+    build-depends: unbuildable<0
+    buildable: False
   hs-source-dirs:    src/
   default-language:  Haskell2010
   if impl(ghc >= 8.0)
@@ -46,6 +49,9 @@ library
 
 
 test-suite doctests-hpath
+  if os(windows)
+    build-depends: unbuildable<0
+    buildable: False
   default-language:  Haskell2010
   type:              exitcode-stdio-1.0
   ghc-options:       -threaded
@@ -57,6 +63,9 @@ test-suite doctests-hpath
                    , hpath
 
 test-suite doctests-posix
+  if os(windows)
+    build-depends: unbuildable<0
+    buildable: False
   default-language:  Haskell2010
   type:              exitcode-stdio-1.0
   ghc-options:       -threaded
@@ -70,6 +79,9 @@ test-suite doctests-posix
                      QuickCheck
 
 test-suite spec
+  if os(windows)
+    build-depends: unbuildable<0
+    buildable: False
   Type:                 exitcode-stdio-1.0
   Default-Language:     Haskell2010
   Hs-Source-Dirs:       test