mingw-fetch-dependencies.sh: Add a wrapper for pkg-config

mingw-fetch-dependencies.sh: Add a wrapper for pkg-config

On a Fedora system (and maybe others) there is a wrapper script called
i686-pc-mingw32-pkg-config. This script unsets the PKG_CONFIG_PATH
variable and then sets the PKG_CONFIG_LIBDIR variable so that it won't
pick up the native system .pc files. This breaks cross compiling in
mingw-fetch-dependencies.sh because it ends up removing its attempts
to set a local search path.

To fix this, the mingw-fetch-dependencies script now generates its own
wrapper script which instead sets PKG_CONFIG_LIBDIR to the local
clutter-cross prefix and then runs the original pkg-config program
from the search path. This should have the same benefit of preventing
it from finding native system .pc files on systems that don't provide
a cross pkg-config. The cross compiling for json-glib and the
recommend args to pass to configure when building clutter are updated
to set the PKG_CONFIG varible to point to this wrapper script.