Hi magsilva.
Just as a reference for future users (and, quite likely, myself), let me leave a record of how to compile this project for M1 and M2 mac users.
/opt/homebrew/Cellar/gsl/2.7.1
/opt/homebrew/Cellar/gflags/2.2.2
# Line 8 in the Makefile
CXXFLAGS += -I /opt/homebrew/Cellar/gsl/2.7.1/include -L /opt/homebrew/Cellar/gsl/2.7.1/lib -I /opt/homebrew/Cellar/gflags/2.2.2/include -L /opt/homebrew/Cellar/gflags/2.2.2/lib -I ${LIB} -I ${LIB}/math -I ${GSLWRAP_LIB}/include -I ${GSLWRAP_LIB}/include/gslwrap
This change allows you to compile in the new architectures.