Honoring --prefix for include

[Sebastien Carpe <scarpe atos-group com>]

It is of a common use to hade $(includedir) in the include directory
list. Would you please fix things so that includedir is honored ?

It is really annoying for us people that can't write things to
/usr/local or so to have to edit each Makefile to add the
-I$(includedir) within. Well, i'm not sure at the moment, but would
you also check that -L$(libdir) is also present ?
Currently, the only thing about prefix i'm 100% sure is honored is the 
installation path.
Thanx for attention and any consideration of this remark :)
-- 
Seb C. (mailto:scarpe@atos-group.com) | Working for Atos at Lille, France
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