When trying to build any qmake base project with NDK r18 qmake fails with the following error message:

clang++: error: unknown argument: '-mthumb-interwork'
Project ERROR: failed to parse default search paths from compiler output

Note: The clang++: ... line is only displayed when compiling for armv7. For the x86 builds, only the second line is displayed.

When inspecting the new NDK files, I found that the gcc/g++ compilers where replaced by shellscripts that invoke clang! See:

>>> cat $ANDROID_NDK_ROOT/toolchains/x86-4.9/prebuilt/linux-x86_64/bin/i686-linux-android-gcc
#!/bin/bash
exec `dirname $0`/../../../../llvm/prebuilt/linux-x86_64/bin/clang -target i686-linux-android "$@"

I checked all the different plattforms, all look the same. So it seems, google has finally removed g++ support...

The prebuild android binaries from the Qt installer where used. This is most likely related to QTBUG-70631, as the error there is also related to the NDK r18.