streamline libdl detection and linking

instead of having a library and a test, use a library with two sources,
the first being empty (i.e., just libc). this allows us doing away with
the "libdl" feature, and using just the "dlopen" one.

subsequently, replace all LIBS+=$$QMAKE_LIBS_DYNLOAD with
QMAKE_USE+=libdl.
the definitions of QMAKE_LIBS_DYNLOAD remain in the qmakespecs for
backwards compat only. n.b.: the only specs where it is not empty or
"-ldl" (i.e., what we support now) are the hpux ones, where the library
is called 'dld'.

technically, the "library" feature should depend on '!unix || dlopen', but
that's for a later patch.

Change-Id: Ib8546affc4b7bc757f1a76729573ddd00e152176
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

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