build: Store tarball URLs in spec files

RPM only cares about the basename part of a Source: filename,
so we can store the original download URL there as long as
the downloaded file has the correct name.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
build: Remove unnecessary build directory name overrides

If the source tarball expands to produce a directory called
%{name}-%{version}, RPM will automatically find it.   -n is only needed
when the directory produced by the tarball has a name which doesn't fit
this pattern.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
build: Move tarball downloading into the makefile

With source URLs now embedded in the spec files, we can generate
rules to download the tarballs needed to build each SRPM.

Signed-off-by: Euan Harris <euan.harris@citrix.com>