[CMake] Detecting compilation option and flags

[Lars Christensen]

I rely on these to detect options and flags:

$<TARGET_PROPERTY:${_target},COMPILE_FLAGS>
$<TARGET_PROPERTY:${_target},COMPILE_OPTIONS>

But not all options are picked up. These are missing:

1. Debug flag (when CMAKE_BUILD_TYPE=Debug)
2. -std=xx (CMAKE_CXX_STANDARD)

How can I reliably detect ALL compilation flags for a target?

Picking up these two flags is required for my pre-compiled header cmake
hack[1]. It used to work OK. I guess Debug can be detected and picked up
from _FLAGS_DEBUG, but I don't see a way to pick up -std=.

Lars

1) https://github.com/larsch/cmake-precompiled-header/
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