Take CMAKE_CXX_FLAGS into account when detecting libcxx

[robinlinden]

Hi! We're building for some obscure embedded platforms using sysroots and ran into an issue where conan fails to find string when trying to autodetect libcxx as it doesn't care about the CXX_FLAGS (where we pass --sysroot).

Just passing ${CMAKE_CXX_FLAGS} doesn't work since the compiler will get "-m32 --sysroot=..." as 1 argument instead of two, so I had to split them using separate_arguments.

[CLAassistant]

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[robinlinden]

If you want to be more selective in which flags are used, I'm fine with changing it so that it only grabs --sysroot from CMAKE_CXX_FLAGS.

[smessmer]

This should probably also set the libcxx variable in PARENT_SCOPE, see #256

[czoido]

Hi @robinlinden,
Thanks a lot for reporting and the contribution. I think this issue will be probably solved by #197 that is marked for review for version 0.16.
I'm going to close this as is duplicate but please feel free to reopen if you think it's necessary.

[puetzk]

#197 doesn't parse it out of CXX_FLAGS (and I don't really think it should try), but it does handle sysroots if you set it the proper CMake way -- either -D or a toolchain which sets CMAKE_SYSROOT or CMAKE_SYSROOT_COMPILE/CMAKE_SYSROOT_LINK (if they need to be separate for some reason)