[Pkg-cmake-team] Bug#945825: [cmake] Can't find development files for Python 3.7

[Ole Streicher]

Package: cmake
Version: 3.15.4-1
Control: affects -1 src:casacore
Severity: important

Dear maintainer,

when both Python 3.7 and 3.8 are installed, cmake does not find the
development files of Python 3.7 and fails if not 3.8 development files
are installed. Specifically, when I try to build casacore, with the
following build dependencies:

Build-Depends: [...], python3-dev, python3-numpy, [...]

Here, python3-dev pull the Python 3.7 development package, but
python3-numpy also depends on Python 3.8 (see bug #945824).

When one now uses cmake's

 find_package(Python3 REQUIRED COMPONENTS Interpreter Development NumPy)

to get the required Python 3 components (of the default Python package,
which is 3.7), the resulting error is

-------------------------------8<-----------------------------------
CMake Error at
/usr/share/cmake-3.15/Modules/FindPackageHandleStandardArgs.cmake:137
(message):
  Could NOT find Python3 (missing: Python3_LIBRARY Python3_INCLUDE_DIR
  Development NumPy) (found version "3.8.0")
Call Stack (most recent call first):
  /usr/share/cmake-3.15/Modules/FindPackageHandleStandardArgs.cmake:378
(_FPHSA_FAILURE_MESSAGE)
  /usr/share/cmake-3.15/Modules/FindPython/Support.cmake:1653
(find_package_handle_standard_args)
  /usr/share/cmake-3.15/Modules/FindPython3.cmake:215 (include)
  python3/CMakeLists-cmake3.14.txt:3 (find_package)
  python3/CMakeLists.txt:4 (include)
-------------------------------8<-----------------------------------

This currently breaks the build of the casacore package for the standard
Python version (and it looks that this will always happen when the
standard version is not the newest supported one).

Best regards

Ole