pass down $CC and $LIBS ($CFLAGS is already used), add ictce easyconfigs for BLAT

depends on https://github.com/hpcugent/easybuild-easyblocks/pull/423
the `libpng` version differs, there's already a `BLAT-3.5-ictce-5.5.0.eb` file (which uses libpng 1.6.2 like the `goolf` one, and which already uses `buildopts`)