[gentoo-dev] [RFC] Enable CMAKE_WARN_UNUSED_CLI by default in cmake-utils for EAPI>=6

[Maciej Mrozowski <reavertm@gmail.com>]

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Hello,

General advise: do not convert ebuilds inheriting cmake-utils to EAPI 6 unless 
you know what you are doing (you are fully aware of eclass behaviour removed 
with https://bugs.gentoo.org/show_bug.cgi?id=514384).

Background:

Pre EAPI-6 cmake-utils.eclass contained certain feature to mitigate CMake 
variable case changes done by upstream.
This feature was explicitly removed with 
https://bugs.gentoo.org/show_bug.cgi?id=514384 and no alternative was 
proposed.
It opened new area of possible ebuild regression bugs when switching to EAPI-6 
for ebuilds inheriting cmake-utils.eclass.

Unfortunately there is common misconception, also among developers, that it's 
sufficient to simply replace "${cmake-utils_use_with foo)" with "-DWITH_foo=ON" 
etc.
This is MOST OF THE TIME not the case.
When converting cmake-utils ebuild to EAPI>=6, one needs to consult 
CMakeLists.txt wrt case each variable is written with since CMake is case-
sensitive and WITH_FOO != WITH_foo != WITH_Foo.

Proposal:

CMake allows warning about unused CMake variables passed by CLI. Since this is 
how Gentoo passes ebuild configuration options, it's proposed to enable this 
feature.
Unfortunately it won't fail compilation but at least it gives a chance to spot 
case mismatch when reading build output.

Future thoughts:

For better damage control it's technically possible to extend configure phase 
of cmake-utuls eclass to check mycmakeargs against parsed package buildsystem 
but this might not be very reliable.

regards
MM

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