Merge branch 'releases/1.6' into 'master'

Avoid dependency on C++ compiler when not needed

See merge request !71

CMakeLists.txt

 cmake_minimum_required(VERSION 3.15...3.26)
-project(${SKBUILD_PROJECT_NAME} LANGUAGES CXX)
+
+# Do not set language yet, but only when actually building extensions, to
+# avoid the need for having a working compiler when it's not used.
+project(${SKBUILD_PROJECT_NAME} LANGUAGES)
 
 option(BUILD_EXTENSIONS "Build C++ extensions" OFF)
 
 if(BUILD_EXTENSIONS)
+  # Only now set the language, and trigger the search for a working compiler.
+  enable_language(CXX)
   set(PYBIND11_NEWPYTHON ON)
   find_package(pybind11 CONFIG REQUIRED)