Select SDK and XCode for Build Qt from sources with MacOS

Hi,

I'm attempting to build Qt 5.15 from sources for macOS using a compiler and (macOS) SDK located in a custom location, i.e. not the default installed on the system. The toolchain and SDK to use are located in the directory /MyCustomLocation/toolchains/x64-macos/Toolchains/<Xcode version>/usr/bin/clang and /MyCustomLocation/toolchains/x64-macos/SDKs/<macOS SDK>. To do this I create a custom platform file generic-macos-qmake.conf.in that sets various qmake variables, e.g. QMAKE_CC, QMAKE_CXX, etc.. I then configure Qt with the following command line:

configure -release -qt-zlib -qt-libjpeg -qt-libpng -qt-freetype -qt-pcre -qt-harfbuzz -no-xcb -skip qtandroidextras -skip qtconnectivity -skip qtpurchasing -skip qtwayland -c+std c+14 -ssl -securetransport -sdk /MyCustomLocation/toolchains/x64-macos/SDKs/MacOSX10.15.sdk -webengine-pepper-plugins -webengine-printing-and-pdf -xplatform generic-macos-qmake.conf.in

This worked for previous versions of Qt 5.x, but fails with Qt 5.15, as qmake insists on using the system's compiler under /Applications/Xcode.app and ignores the settings in the platform file. Is it possible to tell the Qt build system which compiler and SDK to use, or must I stuck with the default compiler on the system ?

I really need to be able to select which compiler to use to build Qt 5.15, and using xcode-select to select different Xcode versions doesn't really scale in a multiuser environment.

Greetings,
Thomas