libtheora: fix ARM build

We need to change to the GitHub source because the release tarball is missing relevant files,
c.f. b167cb4a3e

Without this fix, building ffmpeg for e.g. armv7l-hf-multiplatform fails, because the configure script
hits a linker error while trying to find libtheora .

(cherry picked from commit 8ae7fe9fd3)

pkgs/by-name/li/libtheora/package.nix

@@ -1,11 +1,12 @@
 {
   lib,
   stdenv,
-  fetchurl,
+  fetchFromGitHub,
   autoreconfHook,
   libogg,
   libvorbis,
   pkg-config,
+  perl,
   testers,
   validatePkgConfig,
 }:
@@ -14,13 +15,19 @@ stdenv.mkDerivation (finalAttrs: {
   pname = "libtheora";
   version = "1.2.0";
 
-  src = fetchurl {
-    url = "https://downloads.xiph.org/releases/theora/libtheora-${finalAttrs.version}.tar.gz";
-    hash = "sha256-J5MnM5kDtUTCipKurafQ3P0Dl7WcLzaMxpisVvUVkG4=";
+  src = fetchFromGitHub {
+    owner = "xiph";
+    repo = "theora";
+    tag = "v${finalAttrs.version}";
+    hash = "sha256-kzZh4V6wZX9MetDutuqjRenmdpy4PHaRU9MgtIwPpiU=";
   };
 
   patches = lib.optionals stdenv.hostPlatform.isMinGW [ ./mingw-remove-export.patch ];
 
+  postPatch = lib.optionalString stdenv.hostPlatform.isArmv7 ''
+    patchShebangs lib/arm/arm2gnu.pl
+  '';
+
   configureFlags = [ "--disable-examples" ];
 
   outputs = [
@@ -30,17 +37,24 @@ stdenv.mkDerivation (finalAttrs: {
   ];
   outputDoc = "devdoc";
 
-  nativeBuildInputs = [
-    autoreconfHook
-    pkg-config
-    validatePkgConfig
-  ];
+  nativeBuildInputs =
+    [
+      autoreconfHook
+      pkg-config
+      validatePkgConfig
+    ]
+    ++ lib.optionals stdenv.hostPlatform.isArmv7 [
+      # Needed to run lib/arm/arm2gnu.pl for ARM assembly optimizations
+      perl
+    ];
 
   propagatedBuildInputs = [
     libogg
     libvorbis
   ];
 
+  strictDeps = true;
+
   passthru = {
     tests.pkg-config = testers.hasPkgConfigModules {
       package = finalAttrs.finalPackage;