Added support for libav/ffmpeg dual installs (#346)

* [config] Added support for libav/ffmpeg dual installs

Reordered CPPFLAGS/LIBS so that library flags (includes etc) appear
ahead of prefix/global directives and the correct headers/libraries
are checked (if symbols are defined in alternative libraries).
Updated libav checks to use header declaration checks
rather than link checks (so differences between
libav/ffmpeg can be correctly found), and updated code references
to use new defines.  Added support the alternate pkg-config
package names for libav.  Updated ffmpeg vs libav check
to check MICRO version number (>=100 for ffmpeg).  Simplified
resulting configure script by using a function to merge
CPPFLAGS variables.

* [config] Use FORKED_OPTS prefix for libevent_pthreads option

* [config] Add --with-libav option to select libav even if ffmpeg present

src/transcode.c

@@ -33,14 +33,7 @@
 #include <libavutil/time.h>
 #include <libavutil/pixdesc.h>
 
-#ifdef HAVE_LIBAVFILTER
-# ifdef HAVE_LIBAVUTIL_CHANNEL_LAYOUT_H
-#  include <libavutil/channel_layout.h>
-# endif
-# include <libavfilter/avcodec.h>
-#else
-# include "ffmpeg-compat.h"
-#endif
+#include "ffmpeg-compat.h"
 
 #include "logger.h"
 #include "conffile.h"
@@ -408,7 +401,7 @@ encode_write_frame(struct encode_ctx *ctx, AVFrame *filt_frame, unsigned int str
   return ret;
 }
 
-#if defined(HAVE_AV_BUFFERSRC_ADD_FRAME_FLAGS) && defined(HAVE_AV_BUFFERSINK_GET_FRAME)
+#if HAVE_DECL_AV_BUFFERSRC_ADD_FRAME_FLAGS && HAVE_DECL_AV_BUFFERSINK_GET_FRAME
 static int
 filter_encode_write_frame(struct encode_ctx *ctx, AVFrame *frame, unsigned int stream_index)
 {
@@ -863,7 +856,7 @@ close_output(struct encode_ctx *ctx)
   avformat_free_context(ctx->ofmt_ctx);
 }
 
-#ifdef HAVE_AVFILTER_GRAPH_PARSE_PTR
+#if HAVE_DECL_AVFILTER_GRAPH_PARSE_PTR
 static int
 open_filter(struct filter_ctx *filter_ctx, AVCodecContext *dec_ctx, AVCodecContext *enc_ctx, const char *filter_spec)
 {