(1) Add MLINKS support. Submitted by:	hoek
(1) Add MLINKS support.
Submitted by:	hoek

(2) ELF support.
 2a. Variable PORTOBJFORMAT specifies the object format of the system.
     It is passed down to configure/make via CONFIGURE_ENV/MAKE_ENV,
     and is given to generate-plist via PLIST_SUB.
 2b. In PLIST, substitute lines that end with "/libFOO.so.X" with
     "/libFOO.so.X.0".  (This means PLISTs should only list ELF
     libraries.)
Reviewed by:	jb, jdp, hoek, jseger, steve

(3) Perl5-in-system support.  Basically turns USE_PERL5 into a no-op
    if there exists a "/usr/bin/perl5".  Also fix prior breakage by
    dima (${PREFIX} => ${LOCALBASE} in perl5 path).
Reviewed by:	markm (sort of)

(4) Install requirement file as "+REQUIRE" so it will be executed
    correctly by pkg_delete.
Reported by:	Shigeyuki FUKUSHIMA <shige@kuis.kyoto-u.ac.jp>

(5) Do not disable checksum and makesum when NO_EXTRACT is set.
Pointed out by:	hoek and kiri, among other people

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