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 <[email protected]>
Cc: Dylan Baker <[email protected]>
Cc: Emil Velikov <[email protected]>
Cc: Juan A. Suarez Romero <[email protected]>
Cc: Gert Wollny <[email protected]>
Cc: Jon Turney <[email protected]>
Signed-off-by: Andres Gomez <[email protected]>
Reviewed-and-Tested-by: Eric Engestrom <[email protected]>
Reviewed-by: Dylan Baker <[email protected]>
Reviewed-by: Juan A. Suarez <[email protected]>
Reviewed-By: Gert Wollny <[email protected]>
Reviewed-by: Emil Velikov <[email protected]>

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