Rename DEBUG macro to LEG_DEBUG in leg utility

When DEBUG is defined, leg stops with a non-zero exit code after debug output
which breaks the build. So rename the macro to not get leg's debug code
activated by the common DEBUG macro but still keep the functionality with
LEG_DEBUG.
Be less aggressive when checking for the Discount library

Instead of triggering an exception if Discount can not be found,
expect it might not be available.
Build the leg compiler and use it to generate markdown_parser.c during the build process