Fix configuration

VLC_SAVE_FLAGS is intended to be used only once : before any modification of the flags
Do not use $LIBS after it has been overwritten by FFmpeg checks
(cherry picked from commit 18c6e88e0aff698dc8647c244b509bf786d14c59)

configure.ac

@@ -2705,7 +2705,6 @@ dnl Look for a ffmpeg-config (we are on debian )
 dnl Trying with pkg-config
    PKG_CHECK_MODULES(FFMPEG,[libavcodec, libavformat],
     [
-     VLC_SAVE_FLAGS
      CPPFLAGS="${CPPFLAGS} ${FFMPEG_CFLAGS}"
      CFLAGS="${CFLAGS} ${FFMPEG_CFLAGS}"
      AC_CHECK_HEADERS(ffmpeg/avcodec.h libavcodec/avcodec.h)
@@ -2773,7 +2772,6 @@ fi
 dnl Check if we have (required) img_resample() in libavcodec
 if test "${enable_ffmpeg}" != "no"
 then
-     VLC_SAVE_FLAGS
      CFLAGS="${CFLAGS} ${CFLAGS_ffmpeg}"
      LDFLAGS="${LDFLAGS} ${LDFLAGS_ffmpeg}"
      AC_CHECK_LIB(avcodec, img_resample, ,
@@ -2781,6 +2779,9 @@ then
      VLC_RESTORE_FLAGS
 fi
 
+dnl Clean out environment
+LIBS="${LIBS_save}"
+
 dnl
 dnl  ffmpegaltivec plugin
 dnl