encounter a build error

[lee-fan]

After I build the source file.I test the first program provided by QtXlsxWriter website. But there is a error: qtmain_win.cpp: error: undefined reference to `qMain(int, char**)' collect2.exe: error: error: ld returned 1 exit status

but i got a solution from google: after i add one line "undef main" to the source ,it can behave properly.

And I am confused why this happen?