pkg-config Requires.private entries should be comma-separated

While comparing the .pc (pkg-config) files generated by the make and 
meson builds, I noticed that the Requires.private entries use different 
delimiters.  The make build uses spaces, the meson build uses commas. 
The pkg-config documentation says that it should be comma-separated, but 
apparently about half the .pc in the wild use just spaces.

The pkg-config source code acknowledges that both commas and spaces work:

https://github.com/freedesktop/pkg-config/blob/master/parse.c#L273
https://github.com/pkgconf/pkgconf/blob/master/libpkgconf/dependency.c#L286

I think for consistency we should change the make build to use commas 
anyway.  See attached patch.