[REG 4.2 -> 4.3] gcc/g++ not being autodetected from PATH

1. Have MinGW's gcc and g++ in your PATH variable.
   I used MinGW 5.3.0 from Qt's online installer.
2. Start Creator with empty settings, e.g. by passing the parameter -settingspath <empty directory>
3. See the list of compilers in "Tools" -> "Options..." -> "Build & Run" -> "Compilers".
   MSVC-compilers are being autodetected but MinGW is not.

Creator should automatically detect all compilers in PATH. In Creator 4.2, this used to work correctly.