[Bug 763940] New: 'configure' with using system libav always fails with "Uncompatible libavcodec found"

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=763940

            Bug ID: 763940
           Summary: 'configure' with using system libav always fails with
                    "Uncompatible libavcodec found"
    Classification: Platform
           Product: GStreamer
           Version: 1.7.91
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-libav
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: nexfwall at yandex.ru
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

Created attachment 324361
  --> https://bugzilla.gnome.org/attachment.cgi?id=324361&action=edit
Patch

Output from configure:
  checking for LIBAV... yes
  checking for SWSCALE... yes
  checking avi.h usability... no
  checking avi.h presence... no
  checking for avi.h... no
  configure: Using system-installed libav code
  checking whether libav is provided by FFmpeg... no
  configure: error: Uncompatible libavcodec found

Checked configure.ac and found what it checks version based on
<libavcodec/avcodec.h>, and checking LIBAVCODEC_VERSION_MICRO >= 100.

But this is ffmpeg 3.0:
  [nexfwall at VPCYB1S1R-OS00 RPMBuild]$ grep "LIBAVCODEC_VERSION_MICRO"
/usr/include/ffmpeg/libavcodec/version.h
  #define LIBAVCODEC_VERSION_MICRO 102

Also found, what it saves CPPFLAGS to a var, and then appends libav pkg-config
flags. But restore them before checking libavcodec version:
...
  PKG_CHECK_MODULES(SWSCALE, libswscale libavutil)
  saved_CPPFLAGS="$CPPFLAGS"
  CPPFLAGS="$CPPFLAGS $LIBAV_CFLAGS"
  AC_CHECK_HEADERS([avi.h])
  CPPFLAGS="$saved_CPPFLAGS"
  AC_DEFINE([LIBAV_SOURCE], ["system install"], [Describes where the Libav
libraries come from.])
...

But it should restore it only after checking libav provider version. After
this:
...
  AC_LINK_IFELSE([AC_LANG_PROGRAM([[
    #include <libavcodec/avcodec.h>
    ]],[[
        #if LIBAVCODEC_VERSION_MICRO >= 100
        /* FFmpeg uses 100+ as its micro version */
        #else
        #error libav provider should be FFmpeg
        #endif
    ]])], [is_ffmpeg=yes], [is_ffmpeg=no])
  AC_MSG_RESULT([$is_ffmpeg])
...

So here is the patch. Now configure runs properly.

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