add c++11 flags in cmake build system

[weatherhead99]

This patch enforces needing a c++11 compiler from within the CMake build system. Also adds appropriate c++11 flags in a cross platform way (e.g. if building on gcc < 5.0)

[coveralls]

Coverage remained the same at 93.149% when pulling 99f23e9 on weatherhead99:require_cxx_11 into 07c3c94 on rttrorg:master.

[acki-m]

@weatherhead99
There is already code for this, see here:
https://github.com/rttrorg/rttr/blob/master/CMake/config.cmake#L121
And this function to retrieve the latest supported CXX:
https://github.com/rttrorg/rttr/blob/master/CMake/utility.cmake#L529

Does this fix anything? Because, I am setting: set(CMAKE_CXX_STANDARD 14)
When this not works, I will set C++11

The only thing worth adding, would be a cmake error, when even c++11 is not supported. But I have never tested this, whether this works or not.

[weatherhead99]

hmmm, I do see your point. I encountered this the other day when building against GCC 4.8. The library works, all unit tests pass and my project using it has no issues, but it won't built because unless you pass -std=c++11, gcc 4.8 will not compile c++11 constructs. setting CMAKE_CXX_STANDARD will add these flags to compilers where c++11 is not the default. Note, however, your detection of maximum available c++ standard will still report 11 on gcc 4.8, because it is available. It just doesn't add the flags to compiling.

[acki-m]

GCC 4.8, should also use the flag in c++11
See the CI build in travis with GCC 4.9:
https://travis-ci.org/rttrorg/rttr/jobs/358614015#L842
What do you see in cmake when configure your project?
Does it print: "using C++: 11" ?

PS: What about your PR with conan?

[weatherhead99]

it does print Using c++: 11, but does not append -std=c++11 to the build (NOTE: this is gcc 4.8 , not 4.9, where it works without this flag).

Re; Conan, work on the released 0.9.6 has started here: https://github.com/weatherhead99/conan-rttr/tree/v0.9.6 still have some issues with building on some versions of apple-clang and MSVC. As soon as those are fixed, will PR against your repo :-)

[acki-m]

It seems to be not implemented for older compilers, so a CMake issue.
What you have to do is something like this:
guilhermeferreira/paho.mqtt.cpp@eac8a6f

eclipse-paho/paho.mqtt.cpp#70

[acki-m]

@weatherhead99
Any update here?