This fixes a `pkgconf`, observed in pkgconf/pkgconf#209, where the output
of `pkgconf --list-all` would mistakenly print out package names with leading
slashes, e.g.,
```
$ ./pkgconf.exe --list-all
/bzip2 bzip2 - Lossless, block-sorting data compression
/expat expat - expat XML parser
...
```
This interferes with tools like `cabal-install`, which parse the output of
`pkgconf --list-all` (or `pkg-config --list-all`) and assume that each line
begins with an alphanumeric package name.
pkgconf/pkgconf#209 was fixed upstream in pkgconf/pkgconf#210, but it may be
a while for another version of `pkgconf` is released (see
https://github.com/pkgconf/pkgconf/pull/210#issuecomment-776663093).
In the meantime, this incorporates the patch from pkgconf/pkgconf#210 into
`MINGW-packages`.
Instead of normalizing paths at the end, do it when adding them to
to the pc file, to avoid breaking backslashes used for escaping.
Also fix version field validation.
With this the meson test suite passes.
See #7263
When I initially ported things to meson some didn't correctly build in case
the install prefix wasn't the real prefix (/mingw64 vs C:/msys64/mingw64).
While this initially helped it turned out to be very stupid because I fixed
it by stripping the prefix of all install paths. This makes meson behave
differently compared to upstream.
Remove the patch and rebuild everything.
