Fix -Wunicode-homoglyph build failure (#2561)

Fixes #646

This fixes a build failure that tends to happen when building Dhall
using Nix on macOS.  The failure is related to the
`-Wunicode-homoglyph` `cpp` warning, which this change disables.

Author: Gabriella439

dhall/dhall.cabal

@@ -269,7 +269,7 @@ Common common
         CPP-Options:
           -DNETWORK_TESTS
 
-    GHC-Options: -Wall -Wcompat -Wincomplete-uni-patterns
+    GHC-Options: -Wall -Wcompat -Wincomplete-uni-patterns -optP-Wno-unicode-homoglyph
 
 Library
     Import: common