travis: make Meson find the proper llvm-config

travis: make Meson find the proper llvm-config

Travis CI has moved to LLVM 5.0, and meson is detecting automatically
the available version in /usr/local/bin based on the PATH env variable
order preference.

As for 0.44.x, Meson cannot receive the path to the llvm-config binary
as a configuration parameter. See
https://github.com/mesonbuild/meson/issues/2887 and
https://github.com/dcbaker/meson/commit/7c8b6ee3fa42f43c9ac7dcacc61a77eca3f1bcef

We want to use the custom (APT) installed version. Therefore, let's
make Meson find our wanted version sooner than the one at
/usr/local/bin

Once this is corrected, we would still need a patch similar to:
https://lists.freedesktop.org/archives/mesa-dev/2017-December/180217.html

v2: Create the link only to the specificly wanted LLVM version (Gert).

Cc: Eric Engestrom <eric.engestrom@imgtec.com>
Cc: Dylan Baker <dylan@pnwbakers.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Juan A. Suarez Romero <jasuarez@igalia.com>
Cc: Gert Wollny <gw.fossdev@gmail.com>
Cc: Jon Turney <jon.turney@dronecode.org.uk>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-and-Tested-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Reviewed-By: Gert Wollny <gw.fossdev@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>