[Releasing] 4.8.3_rc2 regression: pkg-config broken for mingw

[Mark Brand]

Just noticed a new regression in 4.8.3_rc2.  Commit 
c7d5071afc6c76925c90ff35041dc4eb86511271 breaks pkg-config for MinGW. 
Despite what the commit message says, pkg-config previously worked and 
was useful, particularly for static Qt. Qt itself even installs its .pc 
files.

The mkspec win32-g++-cross has a PKG_CONFIG definition allows pkg-config 
to be used when cross-compiling for MinGW.

regards,

Mark