/usr/share/HepMC/examples/pythia8/GNUmakefile is in hepmc-examples 2.06.09-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# Makefile for building a HepMC PYTHIA8 example
#
# Lynn Garren garren@fnal.gov
#
# main31 and main32 have been copied from PYTHIA 8.157
# you must source either config.csh or config.sh first
#
# ./main31.exe
# ./main32.exe main32.cmnd hepmcout32.dat
#
################################################################################ Define directory paths 
#    You may have to change PYTHIA8_HOME and/or other variables
#
  HepMCdir      = /usr
  HEPMCLOCATION = $(HepMCdir)
  HepMClib      = -lHepMC
  Pythia_LIB	= -lpythia8 -lLHAPDF -lhepmcinterface
################################################################################ Compiler options
#
  CXX           = g++
  INCLUDES 	= -I/usr/include/pythia8
  CXXFLAGS      =  -ansi -pedantic -Wall -g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wshadow -fbounds-check $(INCLUDES)
ifeq "$(CXX)" "g++"
   FLAGS 	= $(DFLG) -fno-second-underscore $(INCDIR)
else
  FLAGS 	= $(DFLG) $(INCDIR)
endif
  LINK_LIBS     = -Wl,-Bsymbolic-functions -Wl,-z,relro 
UNAME = $(shell uname)
ifneq "$(UNAME)" "Darwin"
  LINK_LIBS     += -Wl,-rpath -Wl,$(HepMCdir)/lib
endif
  HDRS          = $(HepMCdir)/include/HepMC/*.h *.h
  EXAMPLES	=  main31.exe main32.exe
###############################################################################
#
.SUFFIXES:      .o .cc .exe
all:	$(EXAMPLES)
main31.exe: main31.o
	$(CXX) $(CXXFLAGS) -o $@ main31.o \
	$(Pythia_LIB) \
	$(HepMClib)
main32.exe: main32.o
	$(CXX) $(CXXFLAGS) -o $@ main32.o \
	$(Pythia_LIB) \
	$(HepMClib)
###############################################################################
#
.cc.o:         $(HDRS) $<
	$(CXX) $(CXXFLAGS) -c $< -o $@
###############################################################################
# gmake clean       removes all garbage from HepMC directories.
# gmake distclean       removes all compiled libraries, executables, +garbage
#
clean:
	rm -f *.o
distclean: 
	$(MAKE) clean --no-print-directory
	rm -f $(EXAMPLES)
	rm -f *.dat
 |