cmake: ${SDL_CMAKE_DEBUG_POSTFIX} needs to be wrapped in quotes.

Otherwise, if it's an empty string, it causes problems.

Fixes Bugzilla #4117.

CMakeLists.txt

@@ -2013,7 +2013,7 @@ add_library(SDL2main STATIC ${SDLMAIN_SOURCES})
 target_include_directories(SDL2main PUBLIC "$<BUILD_INTERFACE:${SDL2_SOURCE_DIR}/include>" $<INSTALL_INTERFACE:include/SDL2>)
 set(_INSTALL_LIBS "SDL2main")
 if (NOT ANDROID)
-  set_target_properties(SDL2main PROPERTIES DEBUG_POSTFIX ${SDL_CMAKE_DEBUG_POSTFIX})
+  set_target_properties(SDL2main PROPERTIES DEBUG_POSTFIX "${SDL_CMAKE_DEBUG_POSTFIX}")
 endif()
 
 if (ANDROID AND HAVE_HIDAPI)
@@ -2047,7 +2047,7 @@ if(SDL_SHARED)
   target_link_libraries(SDL2 ${EXTRA_LIBS} ${EXTRA_LDFLAGS})
   target_include_directories(SDL2 PUBLIC "$<BUILD_INTERFACE:${SDL2_SOURCE_DIR}/include>" $<INSTALL_INTERFACE:include/SDL2>)
   if (NOT ANDROID)
-    set_target_properties(SDL2 PROPERTIES DEBUG_POSTFIX ${SDL_CMAKE_DEBUG_POSTFIX})
+    set_target_properties(SDL2 PROPERTIES DEBUG_POSTFIX "${SDL_CMAKE_DEBUG_POSTFIX}")
   endif()
   if(IOS OR TVOS)
     set_property(TARGET SDL2 APPEND_STRING PROPERTY COMPILE_FLAGS "-fobjc-arc")
@@ -2093,7 +2093,7 @@ if(SDL_STATIC)
   target_link_libraries(SDL2-static ${EXTRA_LIBS} ${EXTRA_LDFLAGS})
   target_include_directories(SDL2-static PUBLIC "$<BUILD_INTERFACE:${SDL2_SOURCE_DIR}/include>" $<INSTALL_INTERFACE:include/SDL2>)
   if (NOT ANDROID)
-    set_target_properties(SDL2-static PROPERTIES DEBUG_POSTFIX ${SDL_CMAKE_DEBUG_POSTFIX})
+    set_target_properties(SDL2-static PROPERTIES DEBUG_POSTFIX "${SDL_CMAKE_DEBUG_POSTFIX}")
   endif()
   if(IOS OR TVOS)
     set_property(TARGET SDL2-static APPEND_STRING PROPERTY COMPILE_FLAGS "-fobjc-arc")