sysutils/minipro: fix build with GCC-based architectures

This port needs new GCC to build on GCC architectures.

Actually respect CC by putting ?= instead of =.

Also respect CFLAGS and remove post-patch step, instead use a separate patch file.

PR: 238515
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20819