...as external/libnumbertext/ExternalProject_libnumbertext.mk tried to do

Change-Id: I8c0deeaa1db31fc0ab5042c7f8a1ced0d65e29ee
Reviewed-on: https://gerrit.libreoffice.org/53806Reviewed-by: László Németh <nemeth@numbertext.org>
Tested-by: Jenkins <ci@libreoffice.org>