Rather than requiring a bit of boilerplate in every meson.build, which is
only documented in a comment in mesoncore.py, use sensible defaults for
sysconfdir, localstatedir and sharedstatedir depending on the prefix.
Fixes#1637
v2:
For clarity, give get_builtin_option_default() a noneIfSuppress argument,
rather than overloading prefix '' and None with special meanings.
This commit adds support for an additional `moc_extra_arguments` keyword.
It becomes especially handy, when `moc`-ed sources conditionally provide
`slots`, depending on compile time macros (i.e. defines).
This allows a CustomTarget to be indexed, and the resulting indexed
value (a CustomTargetIndex type), to be used as a source in other
targets. This will confer a dependency on the original target, but only
inserts the source file returning by index the original target's
outputs. This can allow a CustomTarget that creates both a header and a
code file to have it's outputs split, for example.
Fixes#1470
Jussi Pakkanen