info doc: fix --infodir to respect DESTDIR for 'make install'

info doc: fix --infodir to respect DESTDIR for 'make install'

info files are introduced by commit 3f829d11ff231980383c384df2ae38e369cd8079

This patch helps packagers to avoid problems when install to $DESTDIR

    >>> Starting src_install
    make -j3 bindir=/tmp/paludis/dev-lang-tcc-9999/image//usr/bin libdir=/tmp/paludis/dev-lang-tcc-9999/image//usr/lib tccdir=/tmp/paludis/dev-lang-tcc-9999/image//usr/lib/tcc includedir=/tmp/paludis/dev-lang-tcc-9999/image//usr/include docdir=/tmp/paludis/dev-lang-tcc-9999/image//usr/share/doc/tcc-9999 mandir=/tmp/paludis/dev-lang-tcc-9999/image//usr/share/man install
    mkdir -p "/tmp/paludis/dev-lang-tcc-9999/image//usr/bin"
    install -m755 tcc "/tmp/paludis/dev-lang-tcc-9999/image//usr/bin"
    mkdir -p "/tmp/paludis/dev-lang-tcc-9999/image//usr/share/man/man1"
    install tcc.1 "/tmp/paludis/dev-lang-tcc-9999/image//usr/share/man/man1"
    mkdir -p /usr/share/info
    install  tcc-doc.info "/usr/share/info"
    ACCESS DENIED  open_wr:      /usr/share/info/tcc-doc.info
    install: cannot create regular file `/usr/share/info/tcc-doc.info': Permission denied
    make: *** [install] Error 1
    /usr/libexec/paludis/utils/emake: emake returned error 2

Signed-off-by: Sergei Trofimovich <st@anti-virus.by>