lib: enable doctests on mac again, note workaround, fix link

[ci skip]

hledger-lib/hledger-lib.cabal

@@ -2,7 +2,7 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 22b7806755a6e3d8afa63a7e941273b64188b90a6695b78fa7f59dcb150e19f7
+-- hash: 408bc36237e01b4976cc96ca0444f05937bd517efb0ef378e0d1d4aac76b9e56
 
 name:           hledger-lib
 version:        1.9.99
@@ -232,8 +232,6 @@ test-suite doctests
   if (!impl(ghc >= 8.0))
     build-depends:
         semigroups ==0.18.*
-  if impl(ghc >= 8.4) && os(darwin)
-    buildable: False
   default-language: Haskell2010
 
 test-suite easytests

hledger-lib/package.yaml

@@ -154,10 +154,12 @@ tests:
     dependencies:
     - doctest >=0.8
     - Glob >=0.7
-    # doctest won't run with ghc 8.4 on mac right now, https://github.com/sol/hpack/issues/199
-    when:
-    - condition: impl(ghc >= 8.4) && os(darwin)
-      buildable: false
+    # doctest with ghc 8.4 on mac requires a workaround, but we'll leave it enabled
+    # https://ghc.haskell.org/trac/ghc/ticket/15105#comment:10
+    # https://github.com/sol/doctest/issues/199
+    # when:
+    # - condition: impl(ghc >= 8.4) && os(darwin)
+    #   buildable: false
 
   hunittests:
     main: hunittests.hs