build: Special-case handling of SDL CFLAGS

SDL adds some "special" CFLAGS that interfere with building normal
binaries. Capture those CFLAGS separately and avoid adding them to
the general CFLAGS.

configure

@@ -4782,7 +4782,13 @@ if enabled nvenc; then
     require_cpp_condition nvEncodeAPI.h "NVENCAPI_MAJOR_VERSION >= 6"
 fi
 
-check_pkg_config sdl "sdl >= 1.2.1 sdl < 1.3.0" SDL_events.h SDL_PollEvent
+# SDL is "special" and adds some CFLAGS that should not pollute anything else.
+if enabled avplay; then
+    CFLAGS_SAVE=$CFLAGS
+    check_pkg_config sdl "sdl >= 1.2.1 sdl < 1.3.0" SDL_events.h SDL_PollEvent &&
+    sdl_cflags=$pkg_cflags
+    CFLAGS=$CFLAGS_SAVE
+fi
 
 ! disabled pod2man   && check_cmd pod2man --help     && enable pod2man   || disable pod2man
 ! disabled texi2html && check_cmd texi2html -version && enable texi2html || disable texi2html