INCLUDEDIR should be defined in Makefile.in, and if not, we should notice.

A patch may target multiple products but only one of them needs to
be installed for the patch to succeed. Fixes an Acrobat Reader 9 patch.

For consistency with related functions msi_dup_property and msi_get_property_int
and because there is no ANSI version of these functions.

They only operate on a database and this way we can use them before a package is created.