gallium/targets: explicitly include a dummy.cpp and remove all the LINK mayhem

gallium/targets: explicitly include a dummy.cpp and remove all the LINK mayhem

Explicitly setting the linker variable was required for old and broken
build toolchains. At this point this should no longer be needed, and
setting the sources lists will trigger generation of the correct LINK
variables.

Explicitly include dummy.cpp to use g++ to link the static library which
in most cases is based upon C++ code.

v2: Reword commit message.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>