Explicitly set search paths for libraries build.

The libraries build is prepared to work with binaries not part of the current
working tree. But in this case the default search path surely points to some
other working tree. Even the binaries in this working tree might have been
compiled with non-standard builtin search paths.

Anyway when building the libraries we want always to use the headers from
the current working tree. Therefore we want to set them explicitly. Instead
of setting all in all five paths on the command lines of ca65, cc65 and ld65 I
opted to just set the single environment variable.
Added library reference em_libref to EMD interface.

Occasionally dynamically drivers suffer from not being to refer to
content in the C library. Therefore I added a mechanism to allow
a C library for a certian target to define a symbol that will be
handed over to dynamic drivers for that target. Then the drivers
can use their refernce to that symbol to access content in the C
library.
Added library reference joy_libref to JOY interface.
Added library reference mouse_libref to MOU interface.
Added library reference ser_libref to SER interface.
Added library reference tgi_libref to TGI interface.
Merge remote-tracking branch 'upstream/master'