cad/lepton-eda: respect CFLAGS

Building with base GCC fails with:
cc1: error: -Wuninitialized is not supported without -O

But this port should use the default -O2.

PR: 239311
Approved by: graahnul.grom@gmail.com (maintainer), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D21142