libspng: fix cross compile

libpng is needed to build a test executable, not included in the final
output. Without libpng in buildInputs, we get the following error:
```
libspng-aarch64-unknown-linux-gnu> Run-time dependency libpng found: NO (tried pkgconfig and cmake)
libspng-aarch64-unknown-linux-gnu> Looking for a fallback subproject for the dependency libpng
libspng-aarch64-unknown-linux-gnu> tests/meson.build:28:10: ERROR: Automatic wrap-based subproject downloading is disabled
```

pkgs/development/libraries/libspng/default.nix

@@ -20,7 +20,7 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-BiRuPQEKVJYYgfUsglIuxrBoJBFiQ0ygQmAFrVvCz4Q=";
   };
 
-  doCheck = true;
+  doCheck = stdenv.buildPlatform.canExecute stdenv.hostPlatform;
 
   mesonBuildType = "release";
 
@@ -32,13 +32,15 @@ stdenv.mkDerivation rec {
 
   outputs = [ "out" "dev" ];
 
+  strictDeps = true;
+
   nativeCheckInputs = [
     cmake
-    libpng
   ];
 
   buildInputs = [
     zlib
+    libpng
   ];
 
   nativeBuildInputs = [