c89 is the name of the C language compiler as required by the POSIX
1003.2 standard, while c99 is the name required by the POSIX 1003.1
2001 standard. Both are actually wrappers for gcc, passing it the
options required to make it conform to said standards in addition to
the options passed via the command line.

Both will only accept those options mandated by the respective
standards.