try this
qmake.exe "Windows Mobile 5.0 Pocket PC SDK (ARMV4I).pro"
-win32 -o "Makefile.Windows Mobile 5.0 Pocket PC SDK (ARMV4I)"
note: don't use dirs names with spaces.

installation rules from assistant
...
Uncompress the files into the directory you want Qt installed; e.g. C:\Qt\4.3.5.
Note: The install path must not contain any spaces.
...