From QDevBlog:
Girish Ramakrishnan announces that Qt v4.3 will end the requirement for people compiling or using Qt on MinGW (i.e. the open-source version on Windows) to use the standard Windows command shell rather than a ported Unix shell:
The problem is that the MinGW make mingw32-make starts spawning processes through sh.exe instead of cmd.exe when a sh.exe is found in the PATH. sh.exe, of course, does not understand Windows shell built-in commands and expects the unix path separator / causing compilation to fail.
For 4.3, we have enhanced qmake to detect sh.exe in the PATH and write out a makefile that mingw32-make/sh.exe likes. This means that you can now compile Qt and your programs using the shell of your choice. Other advantages include usage of built-in sh commands and autoconf scripts. It also turns out compilation using sh is way faster than using cmd.exe.