libGL: use headers from glvnd

The glvnd library contains a copy of the OpenGL headers.  By compiling
against the glvnd headers we completely avoid a dependency on Mesa (and
LLVM).  This reduces rebuild time.  It also prevents accidents, such as
where some packages kept a reference to the mesa.drivers output and thus
had all Mesa drivers in their runtime closure.

pkgs/development/libraries/mesa/stubs.nix

@@ -52,7 +52,7 @@ stdenv.mkDerivation {
 
     mkdir -p $dev/{,lib/pkgconfig,nix-support}
     echo "$out ${libglvnd} ${libglvnd.dev}" > $dev/nix-support/propagated-build-inputs
-    ln -s ${mesa.dev}/include $dev/include
+    ln -s ${libglvnd.dev}/include $dev/include
 
     genPkgConfig() {
       local name="$1"
@@ -61,9 +61,9 @@ stdenv.mkDerivation {
       cat <<EOF >$dev/lib/pkgconfig/$name.pc
     Name: $name
     Description: $lib library
-    Version: ${mesa.version}
+    Version: ${libglvnd.version}
     Libs: -L${libglvnd.out}/lib -l$lib
-    Cflags: -I${mesa.dev}/include -I${libglvnd.dev}/include
+    Cflags: -I${libglvnd.dev}/include
     EOF
     }