In an Android top-level CMake build it is necessary to pass ANDROID_STL=c++_shared.
This is not needed in a qtbase-only build.

In a qtbase-only build, QtAutoDetect.cmake is included before the Android toolchain file.
QtAutoDetect.cmake defaults ANDROID_STL to c++_shared, which is the only value we're supporting.

In a top-level build, the Android toolchain file is included before QtAutoDetect.cmake.
The Android toolchain file defaults ANDROID_STL to c++_static, and we get an error message that this value is unsupported.

The loading of the toolchain file is triggered by the first project command CMake encounters.
We must ensure that QtAutoDetect always runs before any project command.