Issue description

For my work on static-haskell-nix I need an overlay that has both .so files and .a files.

This is because I need .as for linking Haskell exes statically, but .so files for executing TemplateHaskell.

Right now pkgsStatic not only adds .a files, but also disables .so files:

• makeStaticBinaries and makeStaticLibraries set --disable-shared
• static.nix uses those
• static.nix disables shared Haskell libs that TH needs

Proposal

I think we should have the following stack of overlays:

pkgsMusl <- pkgsStatic <- pkgsStaticOnly

• pkgsMusl is what it is now: All packages using musl libc
• pkgsStatic should be the one that has .a files added to all libraries (in addition to .so files), and all binaries (including Haskell ones) linked statically where possible
• pkgsStaticOnly should be what pkgsStatic is now: Having only static libraries and binaries, getting rid of .so files.

CC @matthewbauer @Ericson2314 @dtzWill @nmattia