An android-clang mkspec with libc++ should be provided

While the reasons given in QTBUG-57280 are dubious, in my opinion there should still be an official, supported way to build qt5 with clang AND libc++ instead of libstdc++. (even if it is not made default).

Else the old buggy libstdc++ from gcc 4.9 is used, and more and more C++ programs and libraries are not able to compile with it.
While I understand that the size increase with clang VS gcc on android is an issue, I'd rather have big binaries than no binaries at all due to lacking c++ support.

Currently, trying to use the patch in QTBUG-57280 yields the following at configure:

ERROR: Qt requires a compliant STL library.

I added the result of ./configure in the config.log attachment.