Build using lang/gcc* to avoid runtime issues with old -lgcc_s or Clang

Absence of RPATH leads to mypaint loading old libgcc_s.so.1 from base
first which later breaks |import numpy| as libgfortran wants newer version.
On 10.0+ the issue is masked by Clang failing to instantiate some templates.

PR: 188382
PR: 193429
Differential Revision: https://reviews.freebsd.org/D1616
Approved by: bapt (mentor)