Make system image relocatable

[tkf]

The custom system image created by current PyJulia is not relocatable at the moment. One of the reasons is that it embeds the path to julia-py executable:

pyjulia/src/julia/patch.jl

Lines 36 to 38 in cdd97d7

	Base.eval(Base, quote
	julia_exename() = $julia_py
	end)

An easy fix may be to dynamically load julia-py path from an environment variable. However, since different Python versions may not be ABI compatible (e.g., 3.6 and 3.7 have different PyDateTime layout), we need to check the ABI compatibility at run-time. PyCall.__init__ may be the best location to do this check.