Fix detection of pkg-config on all platforms

a) Don't search for pkg-config if we're only cross-compiling
b) Don't unconditionally error out while cross-compiling if the
   specified pkg-config is not found and the dependency is optional
c) Use the pkg-config binary that was found in check_pkgconfig for the
   actual testing
d) Use shutil.which on the found pkg-config only if it finds it.
   Sometimes shutil.which fails to find it, for instance on Windows
   with absolute paths.

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