r71546 broke imagemagick usage in Windows, since cmd.exe takes the variable

r71546 broke imagemagick usage in Windows, since cmd.exe takes the variable
names as the program name.
I'm just not exporting those variables here, but we need to support them in Windows, too.
We could use putenv() but that seems ugly (and risky). A better solution would be to have
wfShellExec() call proc_open() with a modified environment.

A POSIX shell is required to accept variable assignments before the command so we shouldn't
have problems on any UNIX-like system.
http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_09_01