travis: Also set WANT_PYTHON_VERSION on macOS

.travis.yml

@@ -93,6 +93,7 @@ script:
     fi
 
     cmake -DBUILD_METALIBS=$BUILD_METALIBS \
+    -DWANT_PYTHON_VERSION=$(python -V 2>&1 | cut -d' ' -f2) \
     -DCMAKE_{C,CXX}_COMPILER_LAUNCHER=ccache \
     -DCOMPOSITE_SOURCE_EXCLUSIONS="$COMPOSITE_SOURCE_EXCLUSIONS" \
     -DCOMPOSITE_SOURCE_LIMIT=$COMPOSITE_SOURCE_LIMIT \