Specify LDFLAG pointing to hepmc3/lib64 for thepeg configuration
The thepeg built for mac12 is looking for hepmc3 libraries in lib directory while they are installed to lib64. Defining LDFLAGS pointing to lib64.
The thepeg built for mac12 is looking for hepmc3 libraries in lib directory while they are installed to lib64. Defining LDFLAGS pointing to lib64.