math/glm: make it compile with GCC 7

GCC 7 was set as default in r475856

math/glm doesn't recognize GCC 7.x as a valid compiler. Import changes from
dd48b56e44d699a022c69155c8672caacafd9e8a in upstream to update the list of
supported compilers.

As a side effect this makes cad/openvsp build again.

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D16650