fix: Respect CMAKE_INSTALL_PREFIX and BENCHMARK_ENABLE_INSTALL.

[jenskeiner]

Suggested changes to fix #51.

[greptile-apps]

Greptile Summary

This PR fixes two related CMake install issues: the headers install destination was hardcoded to /usr/local (ignoring CMAKE_INSTALL_PREFIX), and there was no way to opt out of installation. The fix adds include(GNUInstallDirs) for standard prefix-relative variables and gates all install() calls behind the BENCHMARK_ENABLE_INSTALL flag (defaulting ON, same variable name as Google Benchmark for compatibility).

• Header destination fixed: DIRECTORY … DESTINATION /usr/local → DESTINATION \"${CMAKE_INSTALL_INCLUDEDIR}\", and trailing slashes are added so the directory contents are placed directly under CMAKE_INSTALL_INCLUDEDIR rather than creating an extra include/ subdirectory level.
• Installation guard added: BENCHMARK_ENABLE_INSTALL (defaults ON, respects -DBENCHMARK_ENABLE_INSTALL=OFF) using the if(NOT DEFINED) / set() pattern, which is the correct idiom for CMake 3.10 compatibility (avoids CMP0077 issues present with option() before 3.13).

Confidence Score: 4/5

Safe to merge — the install logic is correct and the hardcoded path is properly replaced with prefix-relative variables.

The two targeted changes (GNUInstallDirs adoption and BENCHMARK_ENABLE_INSTALL guard) work as intended with no regressions to the build or library targets. The one outstanding gap is that the EXPORT codspeed-targets clause has no matching install(EXPORT …) call, so the CMake package export file is never written at install time, leaving find_package(codspeed) broken for downstream consumers.

core/CMakeLists.txt — the install(TARGETS … EXPORT codspeed-targets) block needs a companion install(EXPORT codspeed-targets DESTINATION …) to complete the exported package setup.

Important Files Changed

Filename	Overview
core/CMakeLists.txt	Adds GNUInstallDirs for prefix-relative install destinations, wraps all install() calls behind BENCHMARK_ENABLE_INSTALL guard, and fixes directory trailing slashes so header contents (not the directory node itself) land in CMAKE_INSTALL_INCLUDEDIR.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[cmake configure] --> B[include GNUInstallDirs\nsets CMAKE_INSTALL_INCLUDEDIR\nCMAKE_INSTALL_LIBDIR\nCMAKE_INSTALL_BINDIR]
    B --> C{BENCHMARK_ENABLE_INSTALL\ndefined?}
    C -- No --> D[set BENCHMARK_ENABLE_INSTALL = ON]
    C -- Yes --> E{BENCHMARK_ENABLE_INSTALL?}
    D --> E
    E -- ON --> F[install DIRECTORY\ninclude/ → CMAKE_INSTALL_INCLUDEDIR]
    F --> G[install TARGETS codspeed\ninstrument_hooks\nEXPORT codspeed-targets]
    E -- OFF --> H[Skip install — no files\ncopied to prefix]

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[greptile-apps]

Missing install(EXPORT) call for codspeed-targets

The EXPORT codspeed-targets clause on install(TARGETS ...) registers the targets into an export set, but without a matching install(EXPORT codspeed-targets DESTINATION ...) call, no codspeed-targets.cmake file is ever written to disk during installation. Downstream projects relying on find_package(codspeed) after cmake --install won't get the imported-target file they need. Adding install(EXPORT codspeed-targets DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/codspeed") inside the same BENCHMARK_ENABLE_INSTALL guard would complete the export setup.

[jenskeiner]

Fair point. I would like to note that I think the dangling export was not introduced by the proposed code changes, but may have made reaching it more likely. Still, I think gating install() and honoring an install prefix are valid fixes. Maybe fixing the dangling export makes sense in another PR?