CMake: add cmake installation; add SIMPLEINI_USE_SYSTEM_GTEST option

[topazus]

The PR does the folllowing things, which will help for packaging in Linux when using CMake:

• Generate the CMake config, version and target files, and install them
• Installation of header file
• Add SIMPLEINI_USE_SYSTEM_GTEST option to choose to use system gtest dependency or not.

[brofield]

I'm happy that SimpleIni is getting some love from the CMake users, but it concerns me that none of these contributions are tested in any way by the build or test harness. What do you think about contributing a change so that build and execute of the test harness uses CMake?

[dacap]

Just making some comments in this PR as I was asked for:

1. This PR has two different changes (probably would be better to have two commits)
2. I agree about SIMPLEINI_USE_SYSTEM_GTEST, it's a good option for Linux platforms
3. I'm not used to all the GNUInstallDirs options, so I cannot make a valid point about them but it looks like something needed to do a "make install"

It would be nice to include some alternative GitHub workflow to test the cmake toolchain.

[topazus]

1. This PR has two different changes (probably would be better to have two commits)

Thanks for your advice, I will split it into two commits.

• I agree about SIMPLEINI_USE_SYSTEM_GTEST, it's a good option for Linux platforms
• I'm not used to all the GNUInstallDirs options, so I cannot make a valid point about them but it looks like something needed to do a "make install"

Yes, use system gtest and use GNUInstallDirs will facilitate to help with Linux packaging. Btw, I am trying to make simpleini package into Fedora official repository. By applying this patch, it seems fine when building RPM package. see build log: https://kojipkgs.fedoraproject.org//work/tasks/1336/110961336/build.log

[brofield]

Thank you for your contribution.

[brofield]

Looking more into the cmake build harness, and I'm wondering why was the SIMPLEINI_USE_SYSTEM_GTEST option necessary? Wouldn't adding the package to the FetchContent_Declare statement be best, see Using Dependencies Guide? i.e. use the installed package and if not available then build it outselves:

include(FetchContent)
FetchContent_Declare(
	googletest
	DOWNLOAD_EXTRACT_TIMESTAMP ON
	URL https://github.com/google/googletest/archive/refs/tags/v1.14.0.zip
	URL_HASH SHA1=0ac421f2ec11af38b0fff0f1992184032731a8bc
	FIND_PACKAGE_ARGS NAMES GTest
	)
FetchContent_MakeAvailable(googletest)