cpp_build: disable LTO to preserve metadata in .data section

[okhsunrog]

Summary

• Fix proc-macro panic "Struct metadata not present in target library file" when building with LTO enabled

Problem

The cpp crate communicates type metadata (sizes, alignments, trait flags) from cpp_build to cpp_macros by embedding a struct with a magic byte pattern in the .data section of the compiled static library. The proc-macro scans the .a file for this pattern using aho_corasick.

When LTO is enabled via environment CXXFLAGS (e.g. -flto=auto), the C++ compiler emits GIMPLE intermediate representation instead of native machine code. The .data section is empty — all content lives in .gnu.lto_* sections. The magic bytes become invisible to the byte-level scan, causing the proc-macro to panic:

-- rust-cpp fatal error --

Struct metadata not present in target library file.
NOTE: Double-check that the version of cpp_build and cpp_macros match

This affects distributions that enable LTO by default in their build systems:

• Arch Linux (makepkg sets -flto=auto via CXXFLAGS)
• Fedora (LTO enabled by default since F33)
• Gentoo (with LTO profiles)

Currently broken downstream packages include LibrePCB (via qttypes/Slint) and anything else using the cpp crate.

Fix

Pass -fno-lto when compiling the generated C++ library, ensuring the metadata struct is always present as raw bytes in .data regardless of environment LTO settings.

Test plan

• Verified that LibrePCB builds successfully on Arch Linux with this fix (previously failed with 697 errors from qttypes proc-macro panics)
• Existing tests should continue to pass since -fno-lto only affects the small generated metadata library, not the final binary linkage