pkgsCross.mingwW64.giflib: fix build

It was failing with:

    x86_64-w64-mingw32-ld: qprintf.o:qprintf.c:(.text+0xb9): undefined reference to `GifErrorString'

and then

    install gif2rgb gifbuild giffix giftext giftool gifclrmp "/nix/store/vcik0mlp8756g2lpmzbjw3v8mhn1529i-giflib-x86_64-w64-mingw32-5.2.1/bin"
    install: cannot stat 'gif2rgb': No such file or directory

pkgs/development/libraries/giflib/default.nix

@@ -29,6 +29,15 @@ stdenv.mkDerivation rec {
       sha256 = "12afkqnlkl3n1hywwgx8sqnhp3bz0c5qrwcv8j9hifw1lmfhv67r";
       extraPrefix = "./";
     })
+  ] ++ lib.optionals stdenv.hostPlatform.isMinGW [
+    # Build dll libraries.
+    (fetchurl {
+      url = "https://aur.archlinux.org/cgit/aur.git/plain/001-mingw-build.patch?h=mingw-w64-giflib&id=4cf1e519bcf51338dc607d23388fca47d71790c0";
+      sha256 = "KyJi3eqH/Ae+guEK6znraZI5+IPImaoYoW5NTkCvjsg=";
+    })
+
+    # Install executables.
+    ./mingw-install-exes.patch
   ];
 
   nativeBuildInputs = lib.optionals stdenv.isDarwin [

pkgs/development/libraries/giflib/mingw-install-exes.patch

@@ -0,0 +1,11 @@
+--- a/Makefile
++++ b/Makefile
+@@ -92,7 +92,7 @@
+ install: all install-bin install-include install-lib install-man
+ install-bin: $(INSTALLABLE)
+ 	$(INSTALL) -d "$(DESTDIR)$(BINDIR)"
+-	$(INSTALL) $^ "$(DESTDIR)$(BINDIR)"
++	$(INSTALL) $(^:=.exe) "$(DESTDIR)$(BINDIR)"
+ install-include:
+ 	$(INSTALL) -d "$(DESTDIR)$(INCDIR)"
+ 	$(INSTALL) -m 644 gif_lib.h "$(DESTDIR)$(INCDIR)"