build.rs: Disable LTO if enabled implictly

[t-8ch]

LTO between Rust and C is complicated. It requires clang and specific invocations configurations for both Rust and C compilations. If the CFLAGS from the environment enable LTO, for example from distribution build tools this can lead to errors. Instead always disable LTO for the C compilation. There is not much to optimize away anyways.

Closes #1444

[briansmith]

How would the user enable LTO, with this patch?

[t-8ch]

In it's current form not at all. Instead we could use -ffat-lto-objects. LTO with GCC won't work, but with clang it can.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.75%. Comparing base (dcc7784) to head (aff02df). Report is 1014 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2289   +/-   ##
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  Coverage   96.75%   96.75%           
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  Files         173      173           
  Lines       20882    20882           
  Branches      487      487           
=======================================
+ Hits        20204    20205    +1     
  Misses        575      575           
+ Partials      103      102    -1     

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[briansmith]

I think the best way forward with this is to just run the ed25519/x25519 benchmarks with and without C LTO enabled and see if it matters.

[briansmith]

Closing this. I don't think we should be overriding the build configuration in build.rs. The person who requested LTO can disable LTO if they wish.