Substitue configure variables in Paths.mk.in

Substitue configure variables in Paths.mk.in

This patch lays the groundwork to convert variables used in Makefiles
to the common automake style, i.e. PREFIX becomes prefix, MANDIR becomes
mandir and so on.

The reason is that configure variables such as mandir expand to
${datarootdir}/man, and datarootdir expands to ${prefix}/share. This
requires extra expansion in configure.ac before assigning to MANDIR.

Special care must be taken when variable substition is done in other
files, such as xencommons.in. All @VARIABLES@ used in these files have
to be the expanded version, or all other variables must be available at
runtime.

This patch by itself changes nothing, but upcoming changes will make use
of the lowercase variables.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>