Allow for compiler versions >= 10

Both clang and gcc development branches have reached version 10. Since we
only parse for a single digit in the major version number, this causes
COMPILER_VERSION to be set to its default of 0.0.0, meaning version checks
fail with these newer compilers.

Reviewed by: emaste
Approved by: markj (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D21413