eclass: Sync qt5-ebuild with main tree

[ghost]

I've a KDE box (and a VM) built with libc++ system-wide. So I can confirm that this eclass is working well as intended.

Synced with the latest version, at the time of writing. Main tree commit: a7c23e549e15582d04a53f4ab7e3ed2a60b0070b

The diff is the following:

--- qt5-build.eclass.old
+++ qt5-build.eclass
@@ -121,7 +121,7 @@
    release) : ${QT5_BUILD_DIR:=${S}} ;; # workaround for bug 497312
 esac

-IUSE="debug test"
+IUSE="debug libcxx test"

 [[ ${PN} == qtwebkit ]] && RESTRICT+=" mirror" # bug 524584
 [[ ${QT5_BUILD_TYPE} == release ]] && RESTRICT+=" test" # bug 457182
@@ -557,7 +557,7 @@
                if tc-is-gcc; then
                    echo -platform linux-g++
                elif tc-is-clang; then
-                   echo -platform linux-clang
+                   echo -platform $(usex libcxx 'linux-clang-libc++' 'linux-clang')
                fi
            fi
        fi)

[BilyakA]

Great, thanks!