Allow .so dynamic libraries on Darwin

[kirelagin]

See commit.

Not tested, because it causes a massive rebuild, and my computer is incompatible with building GHC.

[kirelagin]

For searchability:

Fixes

ld: library not found for -lblas
clang-7: error: linker command failed with exit code 1 (use -v to see invocation)
`cc' failed in phase `Linker'. (Exit code: 1)

[kirelagin]

Oh look, there is already NixOS/nixpkgs#80191.

[hamishmack]

bors try

[iohk-bors]

try

Build succeeded

• buildkite/haskell-dot-nix
• ci/hydra:Cardano:haskell-nix:required

[maksbotan]

Hi.
I can confirm, that this patch helps to build Haskell library that depends on hmatrix package (which in turn depends on blas and lapack).

With haskell.nix master I get errors about not finding -lblas and -llapack, with this patch I do not.

However, linking of executables is still broken.

Please see https://github.com/maksbotan/nix-hmatrix-repro. If I do

nix-build -A nix-hmatrix-repro.components.exes.repro

I get this error:

Preprocessing executable 'repro' for nix-hmatrix-repro-0.1.0.0..
Building executable 'repro' for nix-hmatrix-repro-0.1.0.0..
[1 of 1] Compiling Main             ( Main.hs, dist/build/repro/repro-tmp/Main.o )
Linking dist/build/repro/repro ...
Undefined symbols for architecture x86_64:
  "__gfortran_concat_string", referenced from:
      _dormqr_ in liblapack.a(dormqr.f.o)
ld: symbol(s) not found for architecture x86_64
clang-7: error: linker command failed with exit code 1 (use -v to see invocation)
`cc' failed in phase `Linker'. (Exit code: 1)
builder for '/nix/store/87nxpk1zqgp75pch7s0v8whikz87v6cl-nix-hmatrix-repro-0.1.0.0-exe-repro.drv' failed with exit code 1

Note that to trigger this error the library must use certain functions from liblapack.a. In my case it's triggered with http://hackage.haskell.org/package/hmatrix-0.20.0.0/docs/Numeric-LinearAlgebra.html#v:geig

[angerman]

LGTM