This change adds the correct suffix to debug mode .pc filenames for
MinGW and also to the Qt libraries listed in the `Requires` field.
The filename adjustment fixes the accidental overwriting of release
mode .pc files with the debug mode variant which required the wrong
variant of the libraries when `debug_and_release` is active.
Note that macOS also supports the `debug_and_release' configuration
but may use the regular library names together with DYLD_IMAGE_SUFFIX.
Creation of *_debug.pc files is turned off as they're identical to their
non-debug counterparts.
More info:
- QTBUG-4155
- Qt commit a0d8fb4ac3cb7bafdb39f340055eacee4f957513
