Use a proper install option for this. Now `install_<type>` can directly
override `install` instead of passing a boolean to the string kwarg
`install_dir_<type>`.
generate_gir forces building both the typelib and gir, and some people
only want one or the other (probably only the typelib?) which means
flagging the other as install_dir: false in the same way custom_target
supports.
As this always worked, albeit undocumented, make sure it keeps working.
It's pretty reasonable to allow, anyway.
Fixes https://github.com/mesonbuild/meson/pull/9484#issuecomment-980131791
D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082