PhD in a less desired field To keep the long story short, I kinda have two options for a PhD. 

Option 1

Do a PhD in engineering at a mediocre university in a developing country. Chances at the very top institutions in the same country are not possible due to some uncontrollable constraints. 

I love engineering related study. However, there are some drawbacks. Opportunities here seriosly lack during and after the PhD. Funding will mostly be scarce and I will have to work while studying or lean on my own earnings to sustain my life. Also, postdoc chances even at decent universities in the world are extremely low. The county is corrupt, so without (non-merit based) good contacts, chances in good academic and industrial positions are also minuscule.

Option 2

Do a PhD in social sciences at a respected university in Europe or US. 

Compared to option 1, opportunities and funding seems to be very well

Although the field is social science, some interdisciplinary study opportunities that require engineering skills might be available. Also, the study area I am interested in is a newly flourishing sub-field. But, still the emphasis is on social sciences. I am also interested in social sciences too, but nowhere close to my interest in engineering.

Questions 

Should one choose option1, just for the sake of the love of engineering? 

Or, should one choose option2, as engineering specific love might be temporary or be some kind of short-sighted guess for someone who is not very experienced at research to clearly determine his future interests?
Obviously we can't answer this question explicitly for you, since it's an opinion.  But I would approach it with this thought process, and maybe you would find it helpful too:

First, I would hold each option up against an outside third option of not doing a PhD at all.  Assuming an average outcome, would be willing to chance an engineering PhD if it meant a lack of funding and job opportunities?  Repeat with the second option - would you be willing to suffer through a social science degree you don't necessarily love, just to have better prospects?  Or would you rather do whatever it is you would do without a PhD?

If both of those answers are that you would rather do the PhD, I think the real thought needs to fall on the second option.  A PhD is a huge, often demoralizing undertaking.  You will almost certainly want to quit at times.  And while most fields, as you said, allow some interdisciplinary flexibility, at least at first you will almost certainly be immersed entirely in the field itself.  Do you enjoy this field enough that it could sustain you through down periods?  

Also, European and US programs are often quite different, in particular in that US PhD programs start right out of undergrad, then do several years of coursework that earns you an incidental Masters, followed by your research.  European programs generally require you to have your Masters coursework done already, then you come in to the program just to do your own research.  Make sure you're familiar with the structure of the programs you're interested in - it's possible that might help narrow your choices down some.

I would also add, especially for the engineering program, that you should talk to some people who have gone through it if you haven't already.  Find out first-hand how good or bad the opportunities were.

Good luck!