Fix `-Wnonportable-include-path` workaround

[dan-zheng]

dex.cabal includes some ghc-options for working around a macOS build error reproducible in some environments: repro'd locally by @dan-zheng but not on the macOS CI machines.

Reproducer if all occurrences of -optP-Wno-nonportable-include-path are removed from dex.cabal:

$ make
...
Preprocessing foreign library 'Dex' for dex-0.1.0.0..
Building foreign library 'Dex' for dex-0.1.0.0..
[2 of 5] Compiling Dex.Foreign.Context [PPrint changed]
[3 of 5] Compiling Dex.Foreign.Serialize [Serialize changed]
[4 of 5] Compiling Dex.Foreign.JIT [Export changed]
[5 of 5] Compiling Dex.Foreign.API [Export changed]

/Users/danielzheng/dex-lang/<built-in>:2:10: error:
     warning: non-portable path to file '".stack-work/dist/x86_64-osx/Cabal-3.0.1.0/build/dex/autogen/cabal_macros.h"'; specified path differs in case from file name on disk [-Wnonportable-include-path]
#include ".stack-work/dist/x86_64-osx/Cabal-3.0.1.0/build/Dex/autogen/cabal_macros.h"
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         ".stack-work/dist/x86_64-osx/Cabal-3.0.1.0/build/dex/autogen/cabal_macros.h"
1 warning generated.

We should get down to the bottom of this and find out what causes the error on macOS environments but not others.

---

Relevant workaround PRs: #378, #381

[apaszke]

My guess is that most Linux filesystems are case-sensitive, but APFS has an often used option to be case-insensitive when it comes to file names.