gnu: audacity: Re-add fallback to locate ffmpeg via pkg-config.

This reintroduces the patch that was deleted by
cf5f7a8bf9, adjusting file names.

* gnu/packages/patches/audacity-ffmpeg-fallback.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it here.
* gnu/packages/audio.scm (audacity)[patches]: Use it here.

Change-Id: I848df8a9762216eb2b02afc3843e0e9806db14be
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/local.mk

@@ -960,6 +960,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/ath9k-htc-firmware-gcc.patch		\
   %D%/packages/patches/ath9k-htc-firmware-gcc-compat.patch	\
   %D%/packages/patches/atlas-gfortran-compat.patch		\
+  %D%/packages/patches/audacity-ffmpeg-fallback.patch		\
   %D%/packages/patches/audiofile-fix-datatypes-in-tests.patch	\
   %D%/packages/patches/audiofile-fix-sign-conversion.patch	\
   %D%/packages/patches/audiofile-CVE-2015-7747.patch		\

gnu/packages/audio.scm

@@ -934,6 +934,7 @@ engineers, musicians, soundtrack editors and composers.")
        (file-name (git-file-name name version))
        (sha256
         (base32 "11sjyz6hxsr5dnndkkkiq7arjxvjl1sycl151xq3a3ggakgql3y1"))
+       (patches (search-patches "audacity-ffmpeg-fallback.patch"))
        (modules '((guix build utils)))
        (snippet
         ;; Remove bundled libraries.

gnu/packages/patches/audacity-ffmpeg-fallback.patch

@@ -0,0 +1,61 @@
+From 3c20057d0cbbbed453a692d4dd4589d865808024 Mon Sep 17 00:00:00 2001
+From: Liliana Marie Prikler <liliana.prikler@gmail.com>
+Date: Sat, 29 Jan 2022 10:44:44 +0100
+Subject: [PATCH] Add pkg-config fallback for locating ffmpeg.
+
+Audacity >=3.1.0 no longer links against ffmpeg, but instead dynamically loads
+it.  This dynamic loading mechanism fails to properly locate libraries outside
+of LD_LIBRARY_PATH.
+
+See <https://issues.guix.gnu.org/53591>.
+
+Bugs-added-by: Marius Bakke <marius@gnu.org>
+
+diff --git a/libraries/lib-ffmpeg-support/CMakeLists.txt b/libraries/lib-ffmpeg-support/CMakeLists.txt
+index b8803a1f5..f86559cca 100644
+--- a/modules/mod-ffmpeg/lib-ffmpeg-support/CMakeLists.txt
++++ b/modules/mod-ffmpeg/lib-ffmpeg-support/CMakeLists.txt
+@@ -1,5 +1,7 @@
+
+ if (${_OPT}use_ffmpeg)
++   pkg_check_modules(FFMPEG libavcodec libavformat libavutil)
++
+    set( SOURCES
+       FFmpegTypes.h
+
+@@ -107,6 +109,12 @@ if (${_OPT}use_ffmpeg)
+       list(APPEND DEFINITIONS PRIVATE _DARWIN_C_SOURCE )
+    endif()
+
++   if (FFMPEG_FOUND)
++      pkg_get_variable(LIBAVCODEC_LIBDIR libavcodec libdir)
++      list(APPEND DEFINITIONS PRIVATE
++                  "-DFFMPEG_PC_LIBDIR=\"${LIBAVCODEC_LIBDIR}\"")
++   endif()
++
+    audacity_library( lib-ffmpeg-support "${SOURCES}" "${LIBRARIES}"
+       "${DEFINITIONS}" ""
+    )
+diff --git a/libraries/lib-ffmpeg-support/FFmpegFunctions.cpp b/libraries/lib-ffmpeg-support/FFmpegFunctions.cpp
+index 55d6f676b..46640ef83 100644
+--- a/modules/mod-ffmpeg/lib-ffmpeg-support/FFmpegFunctions.cpp
++++ b/modules/mod-ffmpeg/lib-ffmpeg-support/FFmpegFunctions.cpp
+@@ -246,6 +246,18 @@ struct FFmpegFunctions::Private final
+       if (library->IsLoaded())
+          return library;
+
++#if defined(FFMPEG_PC_LIBDIR)
++      {
++         static const wxString libdir{FFMPEG_PC_LIBDIR};
++         const wxString fullName = wxFileName(libdir, libraryName).GetFullPath();
++
++         auto library = std::make_shared<wxDynamicLibrary>(fullName);
++
++         if (library->IsLoaded())
++            return library;
++      }
++#endif
++
+       return {};
+    }
+ };