[Qgis-developer] Settings custom variables overwrite PATH instead of append to it on Windows

[Arnaud Morvan]

In Options/System,
I've added a custom variable to append to environment PATH with value 
";C:\Program Files (x86)\ffmpeg"
when QGIS restart python is disabled because environment PATH is 
strictly equal to ";C:\Program Files (x86)\ffmpeg"

custom variables append correctly on other variables like COMPUTERNAME

tested on Windows7 64 bits with QGIS 2.2 32 bits standalone installer

Seems to work correctly on Linux

Arnaud