Improve Makefile to make it more standard and so that

everything is built in current directory (where make tool is invoked),
it relies only on one feature not guaranteed by POSIX/SUS, VPATH
(present in BSD and GNU Make), and it provides conventional variables to
provide compiler (CC), compiler flags (CFLAGS), preprocessor flags (CPPFLAGS),
linker flags (LDFLAGS), and additional libraries and objects to link to (LDADD).

Thanks to Aleksej for this mod.