In .prf files, I find many host_build conditions such as:

!host_build|!cross_compile {
use_gold_linker: QMAKE_LFLAGS += $$QMAKE_LFLAGS_USE_GOLD
enable_new_dtags: QMAKE_LFLAGS += $$QMAKE_LFLAGS_NEW_DTAGS
enable_gdb_index: QMAKE_LFLAGS += $$QMAKE_LFLAGS_GDB_INDEX
}

I find host_build is false when I build qt from source on windows. Under what situations is host_build set to true?