In some cases the license-identification header was missing,
so I added that as well. Replaced the remaining headers that
specifically identified "Versions 3.13.7 and higher".
Makefiles and the build system were deliberately excluded.
For compatibility with msi; users can put -Mvar=value into a
xxx_DBFLAGS variable to have msi define that macro when
generating the xxx.db file. This lets mkmf accept the same
DBFLAGS variable that msi does in this case.
