Select GTK version explicitly when configuring Travis CI builds

Don't rely on just a single GTK version being installed on the system,
as we do now, but select the version we want explicitly, just to avoid
bad surprises in the future (even if it doesn't change anything for
now).

build/tools/travis-ci.sh

@@ -68,6 +68,10 @@ case $wxTOOLSET in
         echo 'Configuring...'
 
         wxCONFIGURE_OPTIONS="--disable-optimise $wxCONFIGURE_FLAGS"
+        if [ -n "$wxGTK_VERSION" ]; then
+            wxCONFIGURE_OPTIONS="--with-gtk=$wxGTK_VERSION $wxCONFIGURE_OPTIONS"
+        fi
+
         if [ "$wxUSE_ASAN" = 1 ]; then
             export LSAN_OPTIONS=suppressions=$(pwd)/misc/suppressions/lsan