It is haram (forbidden) to eat pork, as pig flesh is considered impure (najasat).
Forbidden to you is... the flesh of swine... -- Qur'an 5:3
O Muhammad tell them: I did not find in what has been revealed to me anything forbidden to be eaten by one who wishes to eat it, except the meat of an already dead animal, or running blood or the flesh of swine (pork) - for these are unclean... -- Quran 6:145
Indeed the ground will be regarded as impure as a result of coming in contact with pig body parts. However...
And He it is Who sendeth the winds, glad tidings heralding His mercy, and We send down purifying water from the sky. -- Quran 25:48
"We can just wait for the next rain or snowfall to cleanse the ground, so we do not fear from this side anything" --- Abdel Azziz Qaasim Illi, spokesman for Switzerland's Central Islamic Council (discussing a related incident)
It's not a significant obstacle to building a mosque there.
If anyone is considering doing this, please don't harm animals; pouring alcohol on the site would also make it impure, and have the same effect of (a) making Muslims feel unwelcome, and (b) momentarily obstruct the building of a mosque.