Fix install target names

[Som1Lse]

The following changes were made:

• Portions of the install script was moved to src/CMakeLists.txt.
• Targets are exported with names matching their alias.
• foonathan::lexy is also installed.

I've done some basic testing, and it seems to work with my old code that used add_subdirectory, which was the purpose of this change.

This resolves #107.

[foonathan]

Thank you.

[Som1Lse]

I pushed another update.

Turns out LEXY_ENABLE_INSTALL is set after add_subdirectory(src) which messes up the whole install script. So I moved the install script back inside the main CMakeLists.txt file. I didn't catch this before, since it would only break when configuring a fresh build.

On a similar note, GNUInstallDirs wasn't being included inside src/CMakeLists.txt despite being used. This caused a similar bug, which has now been fixed.

I also added an add_alias function which both adds an alias target, and sets EXPORT_NAME. In an ideal world, we would just be able to export the alias targets, but CMake doesn't support this.

[foonathan]

Turns out LEXY_ENABLE_INSTALL is set after add_subdirectory(src) which messes up the whole install script. So I moved the install script back inside the main CMakeLists.txt file. I didn't catch this before, since it would only break when configuring a fresh build.

Good catch!

Thank you.