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

[sicherha]

Does the feature exist in the most recent commit?

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

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.