Fixed Linux build

You probably want to test that this still works on your current build environments, but in most Linux installations, libminiupnpc is under /usr/include/miniupnpc not /usr/include, so I've added an #ifdef WIN32 to ensure that it uses the correct location in net.cpp.

Other than that there's an ambiguous overload error building on Linux in statisticspage.cpp, which is fixed by manually casting the int64_t into a qlonglong (qt 64 bit signed int). Another option could be to set the value to a qlonglong rather than int64_t in the first place, but didn't know if this might have knock-on effects in other places in the code.

2 more fixes are setting share/genbuild.sh and src/leveldb/build_detect_platform's executable bits to on. With them off the build fails entirely, and takes some resetting to get it back to square 1.