As this is not elseif(), the content has no effect.

Rather than changing it to an elseif(), remove the conditional content.
All versions of GNU prior to 5.0 default to C90/89.

Clang-C.cmake has a similar code block which correctly uses
elseif() for setting the default C dialect to C99.  That may have
been updated from a C90 default at some point, so leave the
version condition there in place for now.