fix: build with CMAKE_CXX_STANDARD:STRING=11 on linux

[mayeut]

The CMake test suite doesn't pass when using the default detection mechanism on manylinux2014.
The CMake test suite is now run when building from sources.

This is using c++11 rather than c++14 because it's available on both manylinux1 & manylinux2014.

fixes #221

[mayeut]

I had to disable some tests for musllinux & opened a discussion about that at https://discourse.cmake.org/t/cmake-test-suite-failing-on-alpine-linux/5064
I did not get to the root cause of the failing tests. It looks like a bug in gcc with c++17 in devtoolset-10. Given there's a plan to move to devtoolset-11, finding the exact root cause might not be worse pursuing.
I did not change the required checks since there's been no review yet.

[henryiii]

Travis times out after 2/3 of the tests. Can we reduce there?

[mayeut]

Travis times out after 2/3 of the tests. Can we reduce there?

I missed that. I thought it was only due to "negative credit balance" given the previous build passed without issues.
If it gets to flaky once there are credits, I'll see about getting more cores running the tests (but I guess it won't help in every case as it probably timed out because of side effects of the host doing something else on the side...).
If it doesn't help, we could target some specific tests suite to run but it gets trickier to do a proper selection.

Let's get this in as-is to get the fix in & go on from there then.

[mayeut]

Tests are showing both timeouts & failures with qemu builds. I'll try to address that "quickly" (given the time it takes with QEMU and that it might require a few trials...)