`python/regina/CMakeLists.txt` does not honor CMAKE_INSTALL_PREFIX

[fogti]

when "just" setting CMAKE_INSTALL_PREFIX when invoking cmake, installation of regina-python fails with

CMake Error at python/regina/cmake_install.cmake:54 (file):
  file cannot create directory: /usr/lib/python3.12/site-packages/regina.
  Maybe need administrative privileges.
Call Stack (most recent call first):
  python/cmake_install.cmake:83 (include)
  cmake_install.cmake:50 (include)

when it should actually install into $CMAKE_INSTALL_PREFIX/lib/python3.12/site-packages/regina...

(REGINA_INSTALL_TYPE is set to XDG, but DESTDIR is unset)

[fogti]

By now, I use the following patch to work around this:

diff --git a/python/regina/CMakeLists.txt b/python/regina/CMakeLists.txt
index 1d86fa74c..5263f984a 100644
--- a/python/regina/CMakeLists.txt
+++ b/python/regina/CMakeLists.txt
@@ -42,7 +42,7 @@ IF (${REGINA_INSTALL_TYPE} STREQUAL XDG)
   # Install the module in python's standard site-packages location.
   # However, we do it directly ourselves (not via distutils), since we
   # want the files to show up in cmake's install_manifest.txt.
-  SET(MODULEDIR $ENV{DESTDIR}${Python_SITELIB}/regina)
+  SET(MODULEDIR ${CMAKE_INSTALL_PREFIX}/${Python_SITELIB}/regina)
 ELSE (${REGINA_INSTALL_TYPE} STREQUAL XDG)
   # Install the module directly ourselves.
   SET(MODULEDIR "${PYLIBDIR}/regina")