Codesigning and notarization on macOS

This is painful.  Thankfully most of the pain is now in CMake and a python script.

To build, codesign, and notarized and installer package, CMake needs to be invoked with:

    cd build
    rm -rf *  # optional but recommended
    cmake .. -DBUILD_PACKAGE=ON -DDOWNLOAD_SODIUM=ON -DMACOS_SIGN_APP=ABC123... -DMACOS_SIGN_PKG=DEF456...

where the ABC123... key is a "Developer ID Installer" key and PKG key is a "Developer ID
Application" key.  You have to go through a bunch of pain, pay Apple money, and then read a bunch of
poorly written documentation that doesn't help very much to create these and get them working.  But once you have them
set up in Keychain, you should be able to list your keys with:

    security find-identity -v

and you should see (at least) one "Developer ID Installer: ..." and one "Developer ID Application:
...".  You need both for reasons that only Apple knows.  The former is used to sign the installer
.pkg, and the latter is used to sign everything *inside* the .pkg, and you can't use the same key
for both because Apple designed code signing by marketing committee rather than ask any actual
competent software developers how code signing should work.

Either way, these two values can be specified either by hex value or description string that
`security find-identity -v` spits out.

You also need to set up the notarization parameters; these can either be specified directly on the
cmake command line by adding:

  -DMACOS_NOTARIZE_ASC=XYZ123 -DMACOS_NOTARIZE_USER=me@example.com -DMACOS_NOTARIZE_PASS=@keychain:codesigning-password

or, more simply, by putting them inside a `~/.notarization.cmake` file that will be included if it
exists (and the MACOS_SIGN_* variables are set) -- see below.

These three values here are:

MACOS_NOTARIZE_ASC: 

Organization-specific unique value; this is printed inside (brackets) when you run: `security
find-identity -v`:

    1) 1C75DDBF884DEF3D5927C3F29BB7FC5ADAE2E1B3 "Apple Development: me@example.com (ABC123XYZ9)"

MACOS_NOTARIZE_USER:

Your Apple Developer login.

MACOS_NOTARIZE_PASS:

This should be an app-specific password created for signing on the Apple Developer website.  You
*can* specify it directly, but it is much better to use the magic `@keychain:blah` value, where
'blah' is a password name recorded in Keychain.  To get that in place you run:

    export HISTFILE=''  # for bash: you don't want to store this in your history
    xcrun altool --store-password-in-keychain-item "NOTARIZE_PASSWORD" -u "user" -p "password"

where NOTARIZE_PASSWORD is just some name for the password (I called it 'blah' or
'codesigning-password' above), and the "user" and "password" are replaced with your actual Apple
Developer account device-specific login credentials.

Optionally, put these last three inside a `~/.notarization.cmake` file:

    set(MACOS_NOTARIZE_USER "jagerman@jagerman.com")
    set(MACOS_NOTARIZE_PASS "@keychain:codesigning-password")
    set(MACOS_NOTARIZE_ASC "SUQ8J2PCT7")

Then, finally, you can build the package from the build directory with:

    make package -j4  # or whatever -j makes you happy
    make notarize

The former builds and signs the package, the latter submits it for notarization.  This can take a
few minutes; the script polls Apple's server until it is finished passing or failing notarization.