cmake: add SDL_INSTALL_CMAKEDIR cache variable to override location of cmake config files

CMakeLists.txt

@@ -3346,15 +3346,16 @@ if(NOT SDL2_DISABLE_INSTALL)
 
   ##### Export files #####
   if (WINDOWS AND NOT MINGW)
-    set(PKG_PREFIX "cmake")
+    set(SDL_INSTALL_CMAKEDIR_DEFAULT "cmake")
   else ()
-    set(PKG_PREFIX "${CMAKE_INSTALL_LIBDIR}/cmake/SDL2")
+    set(SDL_INSTALL_CMAKEDIR_DEFAULT "${CMAKE_INSTALL_LIBDIR}/cmake/SDL2")
   endif ()
+  set(SDL_INSTALL_CMAKEDIR "${SDL_INSTALL_CMAKEDIR_DEFAULT}" CACHE STRING "Location where to install SDL2Config.cmake")
 
   include(CMakePackageConfigHelpers)
   configure_package_config_file(SDL2Config.cmake.in "${CMAKE_CURRENT_BINARY_DIR}/SDL2Config.cmake"
     PATH_VARS CMAKE_INSTALL_PREFIX CMAKE_INSTALL_FULL_BINDIR CMAKE_INSTALL_FULL_INCLUDEDIR CMAKE_INSTALL_FULL_LIBDIR
-    INSTALL_DESTINATION ${PKG_PREFIX}
+    INSTALL_DESTINATION "${SDL_INSTALL_CMAKEDIR}"
   )
   write_basic_package_version_file("${CMAKE_CURRENT_BINARY_DIR}/SDL2ConfigVersion.cmake"
     VERSION ${SDL_VERSION}
@@ -3365,7 +3366,7 @@ if(NOT SDL2_DISABLE_INSTALL)
     install(EXPORT SDL2Targets
       FILE SDL2Targets.cmake
       NAMESPACE SDL2::
-      DESTINATION ${PKG_PREFIX}
+      DESTINATION "${SDL_INSTALL_CMAKEDIR}"
     )
     if(ANDROID AND NOT CMAKE_VERSION VERSION_LESS 3.7)
       install(EXPORT_ANDROID_MK SDL2Targets
@@ -3377,7 +3378,7 @@ if(NOT SDL2_DISABLE_INSTALL)
     install(EXPORT SDL2mainTargets
       FILE SDL2mainTargets.cmake
       NAMESPACE SDL2::
-      DESTINATION ${PKG_PREFIX}
+      DESTINATION "${SDL_INSTALL_CMAKEDIR}"
     )
     if(ANDROID AND NOT CMAKE_VERSION VERSION_LESS 3.7)
       install(EXPORT_ANDROID_MK SDL2mainTargets
@@ -3389,7 +3390,7 @@ if(NOT SDL2_DISABLE_INSTALL)
     install(EXPORT SDL2staticTargets
       FILE SDL2staticTargets.cmake
       NAMESPACE SDL2::
-      DESTINATION ${PKG_PREFIX}
+      DESTINATION "${SDL_INSTALL_CMAKEDIR}"
     )
     if(ANDROID AND NOT CMAKE_VERSION VERSION_LESS 3.7)
       install(EXPORT_ANDROID_MK SDL2staticTargets
@@ -3401,7 +3402,7 @@ if(NOT SDL2_DISABLE_INSTALL)
     install(EXPORT SDL2testTargets
       FILE SDL2testTargets.cmake
       NAMESPACE SDL2::
-      DESTINATION ${PKG_PREFIX}
+      DESTINATION "${SDL_INSTALL_CMAKEDIR}"
     )
     if(ANDROID AND NOT CMAKE_VERSION VERSION_LESS 3.7)
       install(EXPORT_ANDROID_MK SDL2testTargets
@@ -3413,7 +3414,7 @@ if(NOT SDL2_DISABLE_INSTALL)
     FILES
       ${CMAKE_CURRENT_BINARY_DIR}/SDL2Config.cmake
       ${CMAKE_CURRENT_BINARY_DIR}/SDL2ConfigVersion.cmake
-    DESTINATION ${PKG_PREFIX}
+    DESTINATION "${SDL_INSTALL_CMAKEDIR}"
     COMPONENT Devel
   )