CMake: put CMake-level ABI settings in CalamaresConfig

- drop the BuildTreeSettings, it was not usefully used
- make CalamaresConfig repeat the WITH_* settings, so that
  consumers can know the ABI offered

CMakeLists.txt

@@ -543,10 +543,6 @@ set( CMAKE_INSTALL_CMAKEDIR "${CMAKE_INSTALL_LIBDIR}/cmake/Calamares" CACHE PATH
 set( CMAKE_INSTALL_FULL_CMAKEDIR "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_CMAKEDIR}" )
 
 export( PACKAGE Calamares )
-configure_file(
-    CalamaresBuildTreeSettings.cmake.in
-    "${PROJECT_BINARY_DIR}/CalamaresBuildTreeSettings.cmake" @ONLY
-)
 configure_package_config_file(
     "CalamaresConfig.cmake.in"
     "${PROJECT_BINARY_DIR}/CalamaresConfig.cmake"

CalamaresBuildTreeSettings.cmake.in

@@ -1,4 +0,0 @@
-set(CALAMARES_INCLUDE_DIRS
-  "@PROJECT_SOURCE_DIR@/src/libcalamares"
-  "@PROJECT_BINARY_DIR@/src/libcalamares"
-)

CalamaresConfig.cmake.in

@@ -62,7 +62,17 @@ set(Calamares_LIBRARIES Calamares::calamares)
 #
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR})
 
+include(CalamaresAddBrandingSubdirectory)
 include(CalamaresAddLibrary)
 include(CalamaresAddModuleSubdirectory)
 include(CalamaresAddPlugin)
-include(CalamaresAddBrandingSubdirectory)
+
+# These are feature-settings that affect consumers of Calamares
+# libraries as well; without Python-support in the libs, for instance,
+# there's no point in having a Python plugin.
+#
+# This list should match the one in libcalamares/CalamaresConfig.h,
+# which is the C++-language side of the same configuration.
+set(Calamares_WITH_PYTHON @WITH_PYTHON@)
+set(Calamares_WITH_PYTHONQT @WITH_PYTHONQT@)
+set(Calamares_WITH_QML @WITH_QML@)

src/libcalamares/CalamaresConfig.h.in

@@ -1,5 +1,5 @@
 /* === This file is part of Calamares - <https://github.com/calamares> ===
- * 
+ *
  *   SPDX-FileCopyrightText: 2020 Adriaan de Groot <groot@kde.org>
  *
  *   Calamares is free software: you can redistribute it and/or modify
@@ -29,7 +29,14 @@
 #define CMAKE_INSTALL_FULL_DATADIR "${CMAKE_INSTALL_FULL_DATADIR}/calamares"
 #define CMAKE_INSTALL_FULL_SYSCONFDIR "${CMAKE_INSTALL_FULL_SYSCONFDIR}"
 
-//cmakedefines for CMake variables (e.g. for optdepends) go here
+/*
+ * These are feature-settings that affect consumers of Calamares
+ * libraries as well; without Python-support in the libs, for instance,
+ * there's no point in having a Python plugin.
+ *
+ * This list should match the one in CalamaresConfig.cmake
+ * which is the CMake-time side of the same configuration.
+ */
 #cmakedefine WITH_PYTHON
 #cmakedefine WITH_PYTHONQT
 #cmakedefine WITH_QML