Use clang to build as recommended by upstream

According to WebKitGTK 2.46's release blog:

Skia has received several GCC specific optimizations lately, but it’s always more optimized when built with clang. The optimizations are more noticeable in performance when using the CPU for rendering. For this reason, since version 2.46 we recommend to build WebKit with clang for the best performance. GCC is still supported, of course, and performance when built with GCC is quite good too.

Shall we switch to clang to build webkitgtk packages?

added priority3-normal severity4-low statusunconfirmed labels

assigned to @freswa, @toolybird, and @gromit

added scopeenhancement statusconfirmed labels and removed statusunconfirmed label

assigned to @heftig and unassigned @freswa, @toolybird, and @gromit

mentioned in commit webkit2gtk@0144a3a0

mentioned in commit wpewebkit@78e7af81

mentioned in commit webkit2gtk-4.1@46fec6ac

closed with commit b07a7886

mentioned in commit b07a7886