travis: meson: use native files to override llvm-config

This is the supported way to do this, and should be more robust and
reliable.

v2: [Emil]
 - enable backslash escapes
 - don't hardcode the path
 - pass the argument directly to meson

Reviewed-by: Emil Velikov <emil.velikov@collabora.com> (v1)
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

.travis.yml

@@ -650,28 +650,16 @@ script:
         MESON_OPTIONS="-Ddri-drivers=${DRI_DRIVERS:-[]} -Dgallium-drivers=${GALLIUM_DRIVERS:-[]} -Dvulkan-drivers=${VULKAN_DRIVERS:-[]}"
       fi
 
-      # 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.
+      # We need to control the version of llvm-config we're using, so we'll
+      # generate a native file to do so. This requires meson >=0.49
       #
-      # 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
-      test -f /usr/bin/$LLVM_CONFIG && ln -s /usr/bin/$LLVM_CONFIG $HOME/prefix/bin/llvm-config
+      echo -e "[binaries]\nllvm-config = '`which $LLVM_CONFIG`'" > native.file
 
       $LLVM_CONFIG --version
 
       export CFLAGS="$CFLAGS -isystem`pwd`"
-      meson _build $MESON_OPTIONS
+      meson _build $MESON_OPTIONS \
+                   --native-file=native.file
       meson configure _build
       ninja -C _build
     fi