treewide: Fix 'lib.optional' misuses

These add a singleton list of a package to buildInputs.

pkgs/development/libraries/gtk+/3.x.nix

@@ -36,7 +36,7 @@ stdenv.mkDerivation rec {
     [ expat glib cairo pango gdk_pixbuf atk at_spi2_atk
       libXrandr libXrender libXcomposite libXi libXcursor libSM libICE ]
     ++ optionals waylandSupport [ wayland wayland-protocols ]
-    ++ optional stdenv.isDarwin (with darwin.apple_sdk.frameworks; [ AppKit Cocoa ])
+    ++ optionals stdenv.isDarwin (with darwin.apple_sdk.frameworks; [ AppKit Cocoa ])
     ++ optional xineramaSupport libXinerama
     ++ optional cupsSupport cups;
   #TODO: colord?

pkgs/development/libraries/librsvg/default.nix

@@ -18,7 +18,7 @@ stdenv.mkDerivation rec {
   outputs = [ "out" "dev" ];
 
   buildInputs = [ libxml2 libgsf bzip2 libcroco pango libintlOrEmpty ]
-    ++ stdenv.lib.optional enableIntrospection [ gobjectIntrospection ];
+    ++ stdenv.lib.optional enableIntrospection gobjectIntrospection;
 
   propagatedBuildInputs = [ glib gdk_pixbuf cairo ] ++ lib.optional withGTK gtk3;
 

pkgs/development/libraries/openal-soft/default.nix

@@ -21,7 +21,7 @@ stdenv.mkDerivation rec {
   buildInputs = [ cmake ]
     ++ optional alsaSupport alsaLib
     ++ optional pulseSupport libpulseaudio
-    ++ optional stdenv.isDarwin [ CoreServices AudioUnit AudioToolbox ];
+    ++ optionals stdenv.isDarwin [ CoreServices AudioUnit AudioToolbox ];
 
   NIX_LDFLAGS = []
     ++ optional alsaSupport "-lasound"

pkgs/development/libraries/png++/default.nix

@@ -22,7 +22,7 @@ stdenv.mkDerivation rec {
   postCheck = "cat test/test.log";
 
   buildInputs = [ ]
-    ++ stdenv.lib.optional docSupport [ doxygen ];
+    ++ stdenv.lib.optional docSupport doxygen;
 
   propagatedBuildInputs = [ libpng ];