When linking the main tools, the object files to link are set up
via the variable OBJS-<name>, but for the tools, we've only
used the target's list of dependencies.

In most cases, this has been fine, but it has caused specifying
the libraries to link in a duplicate fashion; the linking command
has looked like this:

    $CC -Llibavutil ... tools/tool.o libavutil/libavutil.a -lavutil

Normally, the libraries to link are handled with "-Llibavutil -lavutil";
when linking the main fftools, this is how they are linked.

In the case of the binaries under the "tools" directory (within the
make variable TOOLS), we've passed the full set of dependencies
to the linker, via $^, which does contain the names of the
dependency libraries as well.

When libraries are built as regular static libraries, or shared
unix libraries, this has all worked fine. When libraries are
built as DLLs for Windows, though, the norm is not to pass the
actual DLL to the linker, but an import library.

Mingw tools generally can handle linking directly against a DLL
as well, but MSVC tools don't support that, and error out with
a very cryptic error message:

    libavdevice\avdevice.dll : fatal error LNK1107: invalid or corrupt file: cannot read at 0x2D8

By omitting these parts of the dependencies, linking of these tool
executables succeed in MSVC builds with shared libraries enabled.
Signed-off-by: Martin Storsjö <martin@martin.st>