Makefile: rewrite and simplify

This is how I would write the Makefile.
I believe it is simpler and easier to follow.

I used the implied `CXXFLAGS` and `LDFLAGS` variables
to set the appropriate flags for the C++ compiler and linker.
I also set `CC` to the compiler that will be used, so one can
easily switch to `llvm` or some other compiler.
`CC` should actually be in `config.mak` file, but the makefile
is simple enough to need a such a file separately. 

I also removed the `-static` flag entirely. 
I do not have a statically compiled `boost`.
I don't know if other distros ship one.