[cmake] make sure CXX11 is also always set for addons

[wsnipex]

Description

set cxx11 also for addons

Motivation and Context

fix fallout from #15781

How Has This Been Tested?

only locally compiled on ubuntu 18.04, so not yet verified on affected travis builders

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[ksooo]

Sorry, cannot review due to lack of cmake knowledge and no idea how to runtime-test this.

[AlwinEsch]

From my side OK, but think is not enough.
By this is it only be used on the addons build way over Kodi itself.

If a addon is created independent are this values maybe not present.
By e.g.:
cmake -DADDONS_TO_BUILD=visualization.waveform -DADDON_SRC_PREFIX=../.. -DCMAKE_BUILD_TYPE=Debug -DCMAKE_INSTALL_PREFIX=../../xbmc/kodi-build/addons -DPACKAGE_ZIP=1 ../../xbmc/cmake/addons

This generated the: /build/build/depends/lib/kodi folder for addon with the following files:

• AddonHelpers.cmake
• AddOptions.cmake
• KodiConfig.cmake
• PrepareEnv.cmake

Maybe are the sets done on background, but not sure if everywhere present, the other on Linux/Ubuntu where this is on /usr/lib/kodi is not available.

EDIT: Some are set on KodiConfig.cmake only the CMAKE_ASM_FLAGS not

[AlwinEsch]

Ignore a bit my comment before, think your problem is mainly dependent to the depends build where KodiConfig.cmake becomes not used.

[ksooo]

Is this good to go in?