[BUG] rxvt-unicode 9.31 fails to build with GCC 16 due to lerp() name conflict

[Hemanth Kumar M D]

Hi,

While testing rxvt-unicode 9.31 with GCC 16,
I encountered a build failure caused by a name conflict with 
|std::lerp|, which is available with C++20.

*Issue:*
rxvt-unicode defines a custom |lerp()|function in |src/rxvtutil.h|.
With GCC 16 (C++20), this conflicts with |std::lerp|, leading to 
ambiguous overload resolution.

*Error:*

|rxvttoolkit.C:837:33: error: call of overloaded 'lerp<int, int, 
int>(...)' is ambiguous - candidate: 'constexpr float std::lerp(float, 
float, float)' - candidate: 'T lerp(T, U, P)'|

This matches the issue reported in Gentoo .
https://bugs.gentoo.org/967067

*Fix:*
Rename the custom |lerp()|function to avoid the conflict (e.g., 
|rxvt_lerp()|), and update all call sites accordingly.

This approach has already been applied in Gentoo:
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=785ecf0b5343348e298a5fad9c46ba58b9b303fe 
<https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=785ecf0b5343348e298a5fad9c46ba58b9b303fe>

*Impact:*
This issue affects builds with modern toolchains using GCC 16 (or any 
compiler enabling C++20 |std::lerp|).

-- 
Regards,
Hemanth Kumar M D
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