CMake: Error out when switching sources for dependencies

Currently, configuring from System>Bundled doesn't work, it instead produces cryptic errors. And configuring from Bundled>System wont produce errors, but wont use the system libraries either. This change produces a clear error in both cases.

CMake/DolphinLibraryTools.cmake

@@ -50,6 +50,17 @@ function(dolphin_add_bundled_library library use_system bundled_path)
   add_subdirectory(${bundled_path} EXCLUDE_FROM_ALL)
 endfunction()
 
+function(dolphin_set_library_type library type)
+  if(DEFINED ${library}_TYPE AND NOT ${${library}_TYPE} STREQUAL "${type}")
+    message(FATAL_ERROR
+      "The selection for ${library} changed from '${${library}_TYPE}' to '${type}' after configuration.\n"
+      "Please delete the build directory (or at least CMakeCache.txt) and reconfigure."
+    )
+  endif()
+
+  set(${library}_TYPE "${type}" CACHE INTERNAL "")
+endfunction()
+
 function(dolphin_find_optional_system_library library bundled_path)
   dolphin_optional_system_library(use_system ${library})
   string(TOUPPER ${library} upperlib)
@@ -101,11 +112,11 @@ function(dolphin_find_optional_system_library library bundled_path)
     endif()
   endif()
   if(${prefix}_FOUND)
+    dolphin_set_library_type(${library} "System")
     message(STATUS "Using system ${library}")
-    set(${prefix}_TYPE "System" PARENT_SCOPE)
   else()
+    dolphin_set_library_type(${library} "Bundled")
     dolphin_add_bundled_library(${library} ${use_system} ${bundled_path})
-    set(${prefix}_TYPE "Bundled" PARENT_SCOPE)
   endif()
 endfunction()
 
@@ -119,11 +130,11 @@ function(dolphin_find_optional_system_library_pkgconfig library search alias bun
     endif()
   endif()
   if(${library}_FOUND)
+    dolphin_set_library_type(${library} "System")
     message(STATUS "Using system ${library}")
     dolphin_alias_library(${alias} PkgConfig::${library})
-    set(${library}_TYPE "System" PARENT_SCOPE)
   else()
+    dolphin_set_library_type(${library} "Bundled")
     dolphin_add_bundled_library(${library} ${use_system} ${bundled_path})
-    set(${library}_TYPE "Bundled" PARENT_SCOPE)
   endif()
 endfunction()