[FR]: Export a header-only library `GTest::gtest_prod` via CMake

[sicherha]

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Does the feature exist in the most recent commit?

The Bazel infrastructure already provides a dedicated gtest_prod library, which was introduced by commit 105579a6e43908bc88a289d309492eda8be896be.

Unfortunately, the same does not yet exist for CMake.

Why do we need this feature?

User story

As a C++ application developer using CMake, I want GoogleTest's CMake config to provide a dedicated header-only library for gtest_prod.h, so that I can use the FRIEND_TEST() macro in my application code without having to link against the static GTest::gtest library. Only my unit tests should link against GTest::gtest.

Describe the proposal.

The implementation should look roughly like this:

• In googletest/CMakeLists.txt, add a library named gtest_prod of type INTERFACE.
• Include gtest_prod in the list of targets to be installed.
• Add a pkgconf template googletest/cmake/gtest_prod.pc.in that adds the correct include-path to the compiler flags.

This will cause a new target, GTest::gtest_prod, to be added to GTestTargets.cmake.

When a user application subsequently links against GTest::gtest_prod in its CMakeLists.txt, the correct include-path will be set. The user application will not pull in the static gtest library.

Is the feature specific to an operating system, compiler, or build system version?

No; this is a platform-agnostic feature. To the best of my knowledge, it does not pose any specific requirements on the CMake version.