squash! Treat warnings as errors in Travis CI builds

Add wxALLOW_WARNINGS which can be set to prevent this from happening for
the ports where we don't want to avoids warnings anyhow because they
indicate real problems in the code, such as wxDFB.

build/tools/travis-ci.sh

@@ -56,7 +56,7 @@ case $wxTOOLSET in
         ;;
     *)
         echo 'travis_fold:start:configure'
-        export CXXFLAGS=-Werror
+        [ "$wxALLOW_WARNINGS" = 1 ] || export CXXFLAGS=-Werror
         echo 'Configuring...'
         ./configure --disable-optimise --disable-debug_info $wxCONFIGURE_FLAGS || rc=$?
         if [ -n "$rc" ]; then