cmake script modernization

[sherpya]

some complier flags shouldn't be set directly, they are set by cmake (optimization based on build type) or by the toolchain (ios)

"breaking change": default build is not optimized, because it is Debug, as any other cmake project anyway

Tested with:

• Linux: gcc version 12.1.0 (Debian 12.1.0-8)
• Linux: Debian clang version 14.0.6-2
• macOS Intel: Apple clang version 13.1.6 (clang-1316.0.21.2.5)
• Windows 10: Visual Studio Community 2019 (MSVC 19.29.30146.0 and Windows SDK version 10.0.19041.0 to target Windows 10.0.19043)
• iOS: I suppose it work, on a old version I had to remove bitcode/ios target (this patch originates from it)

BTW: there are a lot of type conversion warnings spotted by MSVC

[ponchio]

Thanks for the update!

About the warnings, I am aware, I need to find some time to add a bunch of static_cast and some cleanup.