configure: Don't copy .spec.in files to SPECS

Instead of copying foo.spec.in to the local SPECS directory
and preprocessing it there, read the original file and write
the processed output to SPECS.   This is much tidier.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #6 from euanh/build-improvements

configure: Don't copy .spec.in files to SPECS