cad/meshlab: fix build on powerpc

Switch to GCC because of missing libomp for powerpc:
In file included from /wrkdirs/usr/ports/cad/meshlab/work/meshlab-Meshlab-2020.05/src/common/filterparameter.cpp:28:
In file included from /usr/local/include/vcglib/vcg/math/matrix44.h:33:
/usr/local/include/vcglib/eigenlib/Eigen/Core:247:10: fatal error: 'omp.h' file not found