packages qt qtbase: install pkgconfig files in standard location

Following up from;
https://github.com/mxe/mxe/issues/84
https://github.com/mxe/mxe/issues/181
http://lists.nongnu.org/archive/html/mingw-cross-env-list/2013-07/msg00024.html

Qt5 now has prefixed *.pc files, so installing them side-by-side with Qt4 in the standard pkgconfig directory shouldn't be an issue. The alternative is to simply document:
```sh
export PKG_CONFIG_PATH_i686_pc_mingw32='$(PREFIX)/$(TARGET)/qt/lib/pkgconfig':$PKG_CONFIG_PATH_i686_pc_mingw32
# note the _underscores_ instead of -dashes- in the triplet
```
There's also a workaround for Qt5 [creating *.pc files with debug libs](https://bugreports.qt-project.org/browse/QTBUG-30898), but I'm not sure how correct this is.