Add cmake build tool

[aleksey-nikolaev]

Migrating to CMake could help to build and install libjpeg on MSVC 15.x as simple as on gcc and you could remove any MSVS solution files (and autotools also)

[vbaderks]

I would be able help in providing the initial CMakeLists.txt files.

[thorfdbg]

I suggest to integrate it into your repository and then make a merge request. Would that work?

[vbaderks]

Sure. I can fork your repository, develop there the CMakeLists.txt files and then create a merge request.
I am assuming that the 3 common C++ compilers: gcc, clang and msvc needs to be supported.

What would be the minimum supported C++ standard?

[thorfdbg]

Sorry for the slow reply. C++11 would be more than sufficient for it. Likely even below, i.e. C++03 would likely go.

The most important targets are, as you say, g++ und msvc.

Thanks for looking into this.

[vbaderks]

I have created pull request #50 as initial starting point. It was tested with:

• MSVC 19.28 (Visual Studio 2019 16.8.5 on Windows 10)
• Clang 10.0.0 (Ubuntu 20.04)
• GNU 9.3.0 (Ubuntu 20.04)

The main library is called "jpeg", the cmd-line application is called "jpeg-tool" as names need to be unique in CMake. I didn't
add an option to change the output name of the cmd-line tool, but this is possible. CMake will, by default, prepend the name of a library with "lib".

Note 1: Generating Ninja build files is the easiest way to analyze if all the compiler options are as intended.
Note 2: Clang 10.0.0 will generate some "no case matching constant switch condition '2' " warnings, I could not find a good switch to disable this warning, adding "default" to the switch will probably help, but I explicitly didn't want make any source code changes.
Note 3: BUILD_LIB will be defined when build a shared library. On Linux some changes are needed to config.h to ensure that JPG_EXPORT has the correct value. This needs to be done and an update to the CMakeLists.txt to change the default visibility.