Move all make variables and rules duplicated between gkrellm and
gkrellmd makefiles into a common file that can be included. This
avoids duplication and hopefully avoids the two makefiles getting out
of sync too much. This is also a preparation to ease overriding
toolchain binaries (compiler, linker, pkgconfig) as part of windows
cross compiling.
This change also puts the "ar" tool into a variable that can be
overridden if needed.
Please note that this change uses the GNU Make specific include
statement but AFAICS the makefiles already depend on GNU extensions in
other areas (":=" and "ifdef" are non-standard make syntax).
Stefan Gehn