It was previously impossible to do this:
```
dep.get_pkgconfig_variable(
'foo',
define_variable: ['prefix', '/usr', 'datadir', '/usr/share'],
)
```
since get_pkgconfig_variable mandated exactly two (if any) arguments.
However, you could do this:
```
dep.get_variable(
'foo',
pkgconfig_define: ['prefix', '/usr', 'datadir', '/usr/share'],
)
```
It would silently do the wrong thing, by defining "prefix" as
`/usr=datadir=/usr/share`, which might not "matter" if only datadir was
used in the "foo" variable as the unmodified value might be adequate.
The actual intention of anyone writing such a meson.build is that they
aren't sure whether the .pc file uses ${prefix} or ${datadir} (or which
one gets used, might have changed between versions of that .pc file,
even).
A recent refactor made this into a hard error, which broke some projects
that were doing this and inadvertently depending on some .pc file that
only used the second variable. (This was "fine" since the result was
essentially meaningful, and even resulted in behavior identical to the
intended behavior if both projects were installed into the same prefix
-- in which case there's nothing to remap.)
Re-allow this. There are two ways we could re-allow this:
- ignore it with a warning
- add a new feature to allow actually doing this
Since the use case which triggered this bug actually has a pretty good
reason to want to do this, it makes sense to add the new feature.
Fixes https://bugs.gentoo.org/916576
Fixes https://github.com/containers/bubblewrap/issues/609
398c4b2287