Allow using LICENSE_FILE when a port has multiple licenses all in one file.

[mat]

The other choice is to give out a warning/error.

[jbeich]

The proper support requires adjusting LICENSES_ASK dialog and installing only one file rather than multiple copies of the same file. However, few ports actually have several licenses in one file. In most cases LICENSE_FILE with LICENSE_COMB != single is invalid. Those should be fixed rather than shoved under the carpet.

$ git grep -lF LICENSE_COMB \*/Makefile\* | xargs fgrep -l LICENSE_FILE= | wc -l
     174