travis: make drivers explicit in Meson targets

Like in the autotools target, make the list of drivers to be built in
each of the Meson targets explicit.

This will help to identify missing dependencies and other issues more
easily.

CC: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

.travis.yml

@@ -35,7 +35,9 @@ matrix:
     - env:
         - LABEL="meson Vulkan"
         - BUILD=meson
-        - MESON_OPTIONS="-Ddri-drivers=[] -Dgallium-drivers=[]"
+        - DRI_DRIVERS=""
+        - GALLIUM_DRIVERS=""
+        - VULKAN_DRIVERS="intel,amd"
         - LLVM_VERSION=6.0
         - LLVM_CONFIG="llvm-config-${LLVM_VERSION}"
       addons:
@@ -55,7 +57,9 @@ matrix:
     - env:
         - LABEL="meson loaders/classic DRI"
         - BUILD=meson
-        - MESON_OPTIONS="-Dvulkan-drivers=[] -Dgallium-drivers=[]"
+        - DRI_DRIVERS="i915,i965,r100,r200,swrast,nouveau"
+        - GALLIUM_DRIVERS=""
+        - VULKAN_DRIVERS=""
       addons:
         apt:
           packages:
@@ -463,7 +467,6 @@ matrix:
     - env:
         - LABEL="macOS meson"
         - BUILD=meson
-        - MESON_OPTIONS="-Degl=false"
       os: osx
 
 before_install:
@@ -647,6 +650,14 @@ script:
   - |
     if test "x$BUILD" = xmeson; then
 
+      if test "x$TRAVIS_OS_NAME" == xosx; then
+        MESON_OPTIONS="-Degl=false"
+      fi
+
+      if test "x$TRAVIS_OS_NAME" == xlinux; then
+        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.