Improve documentation for installing dependancies - error installing pcapy

[italovalcy]

Hi,

If we try to install the requirements for ofp_sniffer using pip on a fresh Debian/Ubuntu installation, we get the following error:

# pip install -r docs/requirements.txt
...
Failed to build pcapy
Installing collected packages: PyYAML, python-dateutil, msgpack, termcolor, pyaml, pcapy, netaddr, influxdb, hexdump
    Running setup.py install for pcapy ... error
    ERROR: Command errored out with exit status 1:
     command: /usr/bin/python3 -u -c 'import io, os, sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-12bp3vbv/pcapy_4d7b3afc73b248658b43d9a05a1d04b6/setup.py'"'"'; __file__='"'"'/tmp/pip-install-12bp3vbv/pcapy_4d7b3afc73b248658b43d9a05a1d04b6/setup.py'"'"';f = getattr(tokenize, '"'"'open'"'"', open)(__file__) if os.path.exists(__file__) else io.StringIO('"'"'from setuptools import setup; setup()'"'"');code = f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /tmp/pip-record-jif4eurr/install-record.txt --single-version-externally-managed --compile --install-headers /usr/local/include/python3.7/pcapy
         cwd: /tmp/pip-install-12bp3vbv/pcapy_4d7b3afc73b248658b43d9a05a1d04b6/
    Complete output (9 lines):
    running install
    running build
    running build_ext
    building 'pcapy' extension
    creating build
    creating build/temp.linux-x86_64-3.7
    x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -I/usr/include/python3.7m -c pcapdumper.cc -o build/temp.linux-x86_64-3.7/pcapdumper.o
    unable to execute 'x86_64-linux-gnu-gcc': No such file or directory
    error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
    ----------------------------------------
ERROR: Command errored out with exit status 1: /usr/bin/python3 -u -c 'import io, os, sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-12bp3vbv/pcapy_4d7b3afc73b248658b43d9a05a1d04b6/setup.py'"'"'; __file__='"'"'/tmp/pip-install-12bp3vbv/pcapy_4d7b3afc73b248658b43d9a05a1d04b6/setup.py'"'"';f = getattr(tokenize, '"'"'open'"'"', open)(__file__) if os.path.exists(__file__) else io.StringIO('"'"'from setuptools import setup; setup()'"'"');code = f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /tmp/pip-record-jif4eurr/install-record.txt --single-version-externally-managed --compile --install-headers /usr/local/include/python3.7/pcapy Check the logs for full command output.

After fixing the gcc issue, new issues are raised:

• pcapdumper.cc:10:10: fatal error: Python.h: No such file or directory
• pcapdumper.cc:11:10: fatal error: pcap.h: No such file or directory

Steps to reproduce:

1. Using a fresh Debian/Ubuntu install (e.g., docker image debian:buster)
2. Follow the instructions to install ofp_sniffer in the README.md file

Thus, it would be interesting to update the Installation process documentation (README.md lines 40-47) to also include this low level dependency:

apt-get install build-essential python3-dev libpcap-dev