net/fort: don't depend on gcc10, fix version check

The current version of FORT builds fine with any c11 compiler. There is
no need for USE_GCC.

Patch the build system to correctly define the version of FORT as seen
in the output of "fort --version" and the "User-Agent:" HTTP header. [1]

Approved by: Toni Yannick Kalombo <toni@devboks.com> (maintainer)
Reported by: Mark Tinka <mark@tinka.africa> [1]

(cherry picked from commit 9604608f8a2d458e8fbd0acb5fc492972098cc8e)