Building Qt6 from source on Linux: qsb not found but it's there

[ale.re]

Hello,
I'm trying to build Qt6 for Android on Linux, but I'm having a few issues I'm not sure how to solve.
In brief: I have a working host Qt6 installation from my distribution, I have a local Qt6 repo with matching checked-out version (v6.3.1), I have the required android tools downloaded and installed, qtcreator has green check marks all around it. Now, I just have to build Qt6 for Android. Here be dragons.

My distribution packages Qt6 modules separately, so Qt6Base is in /opt/foo/qtbase, Qt6ShaderTools is in /opt/bar/qtshadertools and so on.

The problem is that cmake cannot find Qt6ShaderTools, and in particular the qsb tool; but it is installed, in path and discoverable via pkg-config:

$ which qsb
/usr/bin/qsb

$ qsb
Usage: qsb [options] file
Qt Shader Baker (using QShader from Qt 6.3.1)
<cut>

$ pkg-config --list-all | grep Shader
Qt6ShaderTools            Qt6 ShaderTools - Qt ShaderTools module

So, when I try to configure Qt6, I'm getting errors related to missing qsb, I tried using:

$ ../qt6/configure -qt-host-path /opt/foo/qtbase \
                  -platform android-clang -prefix $PWD/../qt6-install \
                  -android-sdk ../android-sdk/ \
                  -android-ndk ../android-sdk/ndk/21.3.6528147/
<cut>
Configuring submodule 'qtbase'
-- Could NOT find double-conversion (missing: double-conversion_DIR)
-- Could NOT find md4c (missing: md4c_DIR)
-- Could NOT find double-conversion (missing: double-conversion_DIR)
-- Could NOT find md4c (missing: md4c_DIR)
Configuring submodule 'qtimageformats'
Configuring submodule 'qtlanguageserver'
Configuring submodule 'qtshadertools'
-- Could NOT find Qt6ShaderToolsTools (missing: Qt6ShaderToolsTools_DIR)
CMake Error at qtbase/cmake/QtToolHelpers.cmake:170 (message):
  The tool "Qt6::qsb" was not found in the Qt6ShaderToolsTools package.
  Package found: 0
Call Stack (most recent call first):
  qtshadertools/tools/qsb/CMakeLists.txt:8 (qt_internal_add_tool)

This is Ok: since Qt6ShaderTools is installed in a different directory, I could expect that - for some reason - cmake couldn't find it using pkg-config alone (I also tried using the -pkg-config flag to force it). So I specified

$ ../qt6/configure <cut> -- -DQt6ShaderToolsTools_DIR=/opt/bar/qtshadertools

in the configure script, so that the flag should be passed to CMake, and indeed it is. This is the cmake command that is invoked when I specify that flag (made readable)

'/opt/baz/cmake' '-DQT_HOST_PATH=/opt/foo/qtbase/'
                 '-Dmd4c_DIR=/opt/qux/md4c/'
                 '-DQt6ShaderToolsTools_DIR=/opt/bar/qtshadertools/'
                 '-DCMAKE_INSTALL_PREFIX=/home/my_project/qt6-android-build/../qt6-android-install'
                 '-DQT_QMAKE_TARGET_MKSPEC=android-clang'
                 '-DANDROID_SDK_ROOT=../android-sdk'
                 '-DANDROID_NDK_ROOT=../android-sdk/ndk/21.3.6528147'
                 '-G' 'Ninja' '/home/my_project/qt6'

still, I get the same errors seen above

Configuring submodule 'qtbase'
-- Could NOT find double-conversion (missing: double-conversion_DIR)
-- Could NOT find md4c (missing: md4c_DIR)
-- Could NOT find double-conversion (missing: double-conversion_DIR)
-- Could NOT find md4c (missing: md4c_DIR)
Configuring submodule 'qtimageformats'
Configuring submodule 'qtlanguageserver'
Configuring submodule 'qtshadertools'
-- Could NOT find Qt6ShaderToolsTools (missing: Qt6ShaderToolsTools_DIR)
CMake Error at qtbase/cmake/QtToolHelpers.cmake:170 (message):
  The tool "Qt6::qsb" was not found in the Qt6ShaderToolsTools package.
  Package found: 0
Call Stack (most recent call first):
  qtshadertools/tools/qsb/CMakeLists.txt:8 (qt_internal_add_tool)

Please note that -DQt6ShaderToolsTools_DIR is set, but when configuring qtshadertools it is reported as missing. Interestingly, this holds true qhen configuring qtbase with md4c_DIR which is installed and it is set in a similar way. double-conversion is also present, but not found by pkg-config.

I'm no cmake expert, but I used -Dblahblah_DIR in past and I expected it to work... The feeling is that such options are kind of ignored when they are passed to qtbase and qtshadertools submodules.

Am I doing something wrong or is the cmake configuration at fault?
Thank you!