CMakeLists.txt: Make c++11 optional unless building tests

[emmenlau]

Several minor changes with respect to c++ standard:

• Changing CMakeLists.txt to make c++11 optional. This is a good idea because SQLiteCpp can support even older compilers. With this change, c++11 becomes requested but the build can proceed if it is not found.
• When tests are enabled, c++11 becomes mandatory. The user is informed about this setting via cmake output.
• Allow users to specify other c++ versions via the cmake options: -DCMAKE_CXX_STANDARD="[98, 11, 14, 17, 20]"

Its my understanding that this PR is more generic than #239 because it allows users to manually specify a newer c++ standard, without all the special logic and extra options.

This PR may also supersede #263 since it is more generic than the other PR.

Comments and feedback welcome.

[coveralls]

Coverage remained the same at 100.0% when pulling b5efb40 on BioDataAnalysis:emmenlau_add_simple_cpp11 into adad71b on SRombauts:master.

[SRombauts]

Thanks, I'll look into it seriously right away.

There is a change needed though: upcoming SQLiteCpp 3.x definitely requires C++11 and will fail with older compilers.
I have started migrating some code to std::unique_ptr<> with use of move constructors & operatos

[emmenlau]

Dear @SRombauts , the newest PR should respect your suggestions. With these changes, the minimum c++ standard will be 11, but other versions are allowed. If the user selects a lower standard than 11 they will get a warning. c++11 or higher standards can be selected unconditionally, and will always be treated as required, so cmake should fail if the compiler does not support it.

[SRombauts]

Thanks a lot, I like it!