Only use -Werror for building, not configuring

Use of -Werror results in failures of configure tests that should
normally succeed, e.g. testing for va_copy() when cross-compiling for
iOS, so don't do it and just enable -Werror for building our own code.

build/tools/travis-ci.sh

@@ -56,7 +56,6 @@ case $wxTOOLSET in
         ;;
     *)
         echo 'travis_fold:start:configure'
-        [ "$wxALLOW_WARNINGS" = 1 ] || export CXXFLAGS='-Werror -Wno-error=cpp'
         echo 'Configuring...'
         ./configure --disable-optimise --disable-debug_info $wxCONFIGURE_FLAGS || rc=$?
         if [ -n "$rc" ]; then
@@ -68,6 +67,8 @@ case $wxTOOLSET in
         fi
         echo 'travis_fold:end:configure'
 
+        [ "$wxALLOW_WARNINGS" = 1 ] || export CXXFLAGS='-Werror -Wno-error=cpp'
+
         echo 'travis_fold:start:building'
         echo 'Building...'
         make -k $wxBUILD_ARGS