python: oh of course, $<TARGET_EXISTS> is too new to be useful.

src/python/CMakeLists.txt

@@ -36,5 +36,21 @@ endif()
 add_subdirectory(corrade)
 add_subdirectory(magnum)
 
+# This would be bad enough already, but $<TARGET_EXISTS> doesn't exist until
+# CMake 3.12, so I need to do two passes, first replacing variables using
+# configure_file() and then replacing generator expressions with file(GENERATE)
+foreach(target
+    magnum_gl
+    magnum_shaders
+    magnum_platform_egl
+    magnum_platform_glx
+    magnum_platform_glfw
+    magnum_platform_sdl2)
+    if(TARGET ${target})
+        set(${target}_file $<TARGET_FILE:${target}>)
+    endif()
+endforeach()
+configure_file(${CMAKE_CURRENT_SOURCE_DIR}/setup.py.cmake
+               ${CMAKE_CURRENT_BINARY_DIR}/setup.py.in)
 file(GENERATE OUTPUT ${output_dir}/setup.py
-    INPUT ${CMAKE_CURRENT_SOURCE_DIR}/setup.py.cmake)
+    INPUT ${CMAKE_CURRENT_BINARY_DIR}/setup.py.in)

src/python/setup.py.cmake

@@ -33,12 +33,12 @@ extension_paths = {
     # Filled in by cmake
     'corrade.containers': '$<TARGET_FILE:corrade_containers>',
     'magnum._magnum': '$<TARGET_FILE:magnum>',
-    'magnum.gl': '$<$<TARGET_EXISTS:magnum_gl>:$<TARGET_FILE:magnum_gl>>',
-    'magnum.shaders': '$<$<TARGET_EXISTS:magnum_shaders>:$<TARGET_FILE:magnum_shaders>>',
-    'magnum.platform.egl': '$<$<TARGET_EXISTS:magnum_platform_egl>:$<TARGET_FILE:magnum_platform_egl>>',
-    'magnum.platform.glx': '$<$<TARGET_EXISTS:magnum_platform_glx>:$<TARGET_FILE:magnum_platform_glx>>',
-    'magnum.platform.glfw': '$<$<TARGET_EXISTS:magnum_platform_glfw>:$<TARGET_FILE:magnum_platform_glfw>>',
-    'magnum.platform.sdl2': '$<$<TARGET_EXISTS:magnum_platform_sdl2>:$<TARGET_FILE:magnum_platform_sdl2>>',
+    'magnum.gl': '${magnum_gl_file}',
+    'magnum.shaders': '${magnum_shaders_file}',
+    'magnum.platform.egl': '${magnum_platform_egl_file}',
+    'magnum.platform.glx': '${magnum_platform_glx_file}',
+    'magnum.platform.glfw': '${magnum_platform_glfw_file}',
+    'magnum.platform.sdl2': '${magnum_platform_sdl2_file}',
 }
 
 class TheExtensionIsAlreadyBuiltWhyThisHasToBeSoDamnComplicated(build_ext):