Make it build in arch linux and Debian

* Probably unresolved external due to newer gcc, thus
  added libsdl to libui.a as well.
* Make it work for all distros, which either have SDL2::SDL2
  or ${SDL2_LIBRARIES}, provided by sdl2-config or FindSDL2.cmake.

Signed-off-by: Andreas J. Reichel <homebase_ar@web.de>

src/CMakeLists.txt

@@ -81,9 +81,13 @@ if(MINGW)
 	target_link_libraries(86Box SDL2::SDL2-static)
 elseif(WIN32)
 	target_link_libraries(86Box SDL2::SDL2)
+else()
+        if (TARGET SDL2::SDL2)
+                target_link_libraries(86Box SDL2::SDL2)
         else()
 	        target_link_libraries(86Box ${SDL2_LIBRARIES})
         endif()
+endif()
 
 find_package(PNG REQUIRED)
 include_directories(${PNG_INCLUDE_DIRS})

src/unix/CMakeLists.txt

@@ -12,6 +12,18 @@ add_library(plat STATIC ${PLAT_SOURCES} unix_thread.c)
 add_library(ui STATIC unix.c unix_sdl.c unix_cdrom.c)
 target_compile_definitions(ui PUBLIC _FILE_OFFSET_BITS=64)
 target_link_libraries(ui dl)
+
+find_package(SDL2 REQUIRED)
+include_directories(${SDL2_INCLUDE_DIRS})
+if(MINGW)
+	target_link_libraries(ui SDL2::SDL2-static)
+else()
+        if (TARGET SDL2::SDL2)
+                target_link_libraries(ui SDL2::SDL2)
+        else()
+                target_link_libraries(ui ${SDL2_LIBRARIES})
+        endif()
+endif()
 if (ALSA_FOUND)
     target_link_libraries(plat ALSA::ALSA)
 endif()