build: add support for "DESTDIR" variable
build: add support for "DESTDIR" variable

The build system uses "instroot" to define the path for installation
however "DESTDIR" is the most common used one (e.g Debian and
OpenEmbedded use it).

This patch makes "instroot" defaults to "DESTDIR" value if not set.

Signed-off-by: Otavio Salvador <1d31b11c1b92bf5d043d36edfe7174db39a6b891@ossystems.com.br>

Distrotech/newt,Distrotech/newt,Distrotech/newt