Looks like a problem with ruby, try reinstalling ruby, make sure you have installed ALL the required packages, webkit is one of those packages that require a lot of time and memory to build so make sure you have enough available either as real memory or swap space.
Failing that webkitgtk-2.4.8 builds OK, sometimes just going up or down a version can fix a lot of problems.
FYI dependency list for webkitgtk-2.4.8
Code:
keithhedger@LFSCerebro:~-> lfspkg -F webkitgtk-2.4.8
Found dependency pcre-8.36
Found dependency libffi-3.2.1
Found dependency expat-2.1.0
Found dependency pkg-config-0.28
Found dependency Python-2.7.9
Found dependency libxml2-2.9.2
Found dependency UnZip-6.0
Found dependency docbook-xml-4.5
Found dependency docbook-xsl-1.78.1
Found dependency Itstool-2.0.2
Found dependency libxslt-1.1.28
Found dependency xmlto-0.0.26
Found dependency gtk-doc-1.21
Found dependency GLib-2.42.1
Found dependency gobject-introspection-1.42.0
Found dependency GStreamer1.0-1.4.5
Found dependency gst-plugins-base1.0-1.4.5
Found dependency ATK-2.14.0
Found dependency cython-0.19.1
Found dependency yasm-1.3.0
Found dependency libjpeg-turbo-1.4.0
Found dependency libpng-1.6.16
Found dependency LibTIFF-4.0.3
Found dependency gdk-pixbuf-2.31.2
Found dependency Pixman-0.32.6
Found dependency Cairo-1.14.0
Found dependency icu4c-54.1
Found dependency Which-2.20
Found dependency FreeType-2.5.5
Found dependency Harfbuzz-0.9.38
Found dependency Fontconfig-2.11.1
Found dependency util-macros-1.19.0
Found dependency proto-7.7
Found dependency libXau-1.0.8
Found dependency xcb-proto-1.11
Found dependency libxcb-1.11
Found dependency xorg-libs-7.7
Found dependency Pango-1.36.8
Found dependency hicolor-icon-theme-0.14
Found dependency gtk-2.24.26
Found dependency SQLite-3.8.8.2
Found dependency libsoup-2.48.1
Found dependency libdrm-2.4.59
Found dependency elfutils-0.161
Found dependency libvdpau-0.8
Found dependency LLVM-3.5.1
Found dependency MesaLib-10.4.5
Found dependency Ruby-2.2.0
Found dependency libwebp-0.4.2
Found dependency libsecret-0.18
Found dependency webkitgtk-2.4.8
