Compiling under openSUSE Tumbleweed

[Theo]

I compiled Falkon 3.1.99 (dfb3d235) from source because I couldn't find
a package for openSUSE Tumbleweed with Python plugin support (and to
benefit from the latest improvements). I'll describe the steps to my
success and ask some questions about possible bugs (or about what I'm
doing wrong).

Prerequisites that where missing on my system:

* kcrash-devel, purpose-devel (for optional KDE Frameworks Integration
plugin)
* python3-pyside2, python3-pyside2-devel (for optional Python plugins)
* clang-devel, clang7-devel (clang-devel is also version 7, I installed
both just in case) CMake didn't complain about this one missing. Is this
a bug? I installed these packages after 'make' gave this error:

qt.shiboken: (pyfalkon) Unable to locate Clang's built-in include
directory (neither by checking the environment variables
LLVM_INSTALL_DIR, CLANG_INSTALL_DIR  nor running llvm-config). This may
lead to parse errors.
/usr/lib64/gcc/x86_64-suse-linux/9/../../../../include/c++/9/cstddef:50:10:
fatal error: 'stddef.h' file not found

Steps:

$ git clone git://anongit.kde.org/falkon.git
$ cd falkon
$ mkdir build && cd build
$ cmake -DCMAKE_INSTALL_PREFIX=/opt/falkon -DCMAKE_BUILD_TYPE=None ..

Without '-DCMAKE_BUILD_TYPE=None', I get this error from CMake:

CMake Error at
/usr/lib64/cmake/Shiboken2-5.12.3/shiboken_helpers.cmake:160
(shiboken_get_library_for_current_config):
  shiboken_get_library_for_current_config Function invoked with incorrect
  arguments for function named: shiboken_get_library_for_current_config
Call Stack (most recent call first):

/usr/lib64/cmake/Shiboken2-5.12.3/Shiboken2Config.cpython-37m-x86_64-linux-gnu.cmake:48
(shiboken_compute_python_libraries)
  /usr/lib64/cmake/Shiboken2-5.12.3/Shiboken2Config.cmake:5 (include)
  CMakeLists.txt:117 (find_package)

Is this related to https://bugs.kde.org/show_bug.cgi?id=405833 ?

$ make
$ sudo make install
$ mkdir /usr/local/lib64/plugins
$ ln -s /opt/falkon/lib64/plugins/falkon /usr/local/lib64/plugins/falkon
$ export XDG_DATA_DIRS="/opt/falkon/share:$XDG_DATA_DIRS"
$ /opt/falkon/bin/falkon

Falkon looks for plug-ins only under /usr/local/lib64/plugins/falkon,
despite '-DCMAKE_INSTALL_PREFIX=/opt/falkon'. Is this a bug? I also tried

$ export QT_PLUGIN_PATH="/opt/falkon/lib64/plugins:$QT_PLUGIN_PATH"

without success.