configure: don't use SDL.h in check_func_headers when checking for SDL2

check_func_headers() defines a main() function, which clashes with a
redefinition done by said SDL header. Check for SDL_PollEvent using
SDL_events.h only instead, where the redefinition doesn't happen.

Fixes a regression since d03c39b4.

Tested-by: RiCON
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -6010,15 +6010,13 @@ fi
 
 if enabled sdl2; then
     SDL2_CONFIG="${cross_prefix}sdl2-config"
-    if test_pkg_config sdl2 "sdl2 >= 2.0.1 sdl2 < 2.1.0" SDL_events.h SDL_PollEvent; then
-        check_func_headers SDL.h SDL_Init $sdl2_extralibs $sdl2_cflags ||
-            disable sdl2
-    elif "${SDL2_CONFIG}" --version > /dev/null 2>&1; then
+    test_pkg_config sdl2 "sdl2 >= 2.0.1 sdl2 < 2.1.0" SDL_events.h SDL_PollEvent
+    if disabled sdl2 && "${SDL2_CONFIG}" --version > /dev/null 2>&1; then
         sdl2_cflags=$("${SDL2_CONFIG}" --cflags)
         sdl2_extralibs=$("${SDL2_CONFIG}" --libs)
         check_cpp_condition SDL.h "(SDL_MAJOR_VERSION<<16 | SDL_MINOR_VERSION<<8 | SDL_PATCHLEVEL) >= 0x020001" $sdl2_cflags &&
         check_cpp_condition SDL.h "(SDL_MAJOR_VERSION<<16 | SDL_MINOR_VERSION<<8 | SDL_PATCHLEVEL) < 0x020100" $sdl2_cflags &&
-        check_func_headers SDL.h SDL_Init $sdl2_extralibs $sdl2_cflags &&
+        check_func_headers SDL_events.h SDL_PollEvent $sdl2_extralibs $sdl2_cflags &&
             enable sdl2
     fi
     if test $target_os = "mingw32"; then