CMake: Add ALWAYS_INSTALL_HEADERS option

[squeek502]

Closes #457

@teto let me know if this is the sort of thing you had in mind.

[doronbehar]

I'm the original person who wanted to update our luv on NixOS, (see NixOS/nixpkgs#80528).

Currently, without the patch and with BUILD_MODULE=OFF, we get the tree:

qvfnhmpb22wyl5zzxynin07lcxn1qy2x-luv-1.34.1-1/
├── include
│   └── luv
│       ├── lhandle.h
│       ├── lreq.h
│       ├── luv.h
│       └── util.h
├── lib
│   ├── libluv.so -> libluv.so.1
│   ├── libluv.so.1 -> libluv.so.1.34.1
│   ├── libluv.so.1.34.1
│   └── pkgconfig
│       └── libluv.pc
└── nix-support
    └── propagated-build-inputs

5 directories, 9 files

With the patch and with BUILD_MODULE=ON and ALWAYS_INSTALL_HEADERS=ON, we get the tree:

/nix/store/mnv468b533aacm6ji3ywfzlfbg5nr6yc-luv-1.34.1-1
├── include
│   └── luv
│       ├── lhandle.h
│       ├── lreq.h
│       ├── luv.h
│       └── util.h
├── lib
│   └── lua
│       └── 5.1
│           └── luv.so
└── nix-support
    └── propagated-build-inputs

6 directories, 6 files

But with this ^ tree, Neovim (e.g) fails to build because it can't find e.g the pkg-config files as found within the 1st tree.

What's so frustrating about the current behavior is that building the Lua module (that ends up in $prefix/lib/lua/5.1/) disables the installation of (almost I think) anything else.

Ideally, I'd like to see a fix as follows:

• Make BUILD_MODULE=ON install the module but not disable installation of anything else!
• Without any new flag introduced, if BUILD_SHARED_LIBS=ON, the headers, the pkg-config and whatever else that ended up in lib/ in the 1st tree will get installed as well.

[squeek502]

@doronbehar right now BUILD_MODULE and BUILD_SHARED_LIBS are expected to be mutually exclusive (if one is on, the other is expected to be off). Are you wanting to be able to build both at once (and install both at once)?

[doronbehar]

Correct.