unix/configure: Add some libraries needed to detect X11 in some cases
unix/doc: Add a cross-compiling section to readme_unix.html
apu/resampler.h: Make the resampler destructor virtual

Avoids delete-non-virtual-dtor warning
port.h: Make READ_WORD a real function instead of a macro

Well, with some very limited exceptions (like MAX, MIN...), macros are bad.
Playing with pointer arithmetic can be fun but may backfire..
So let the compiler optmize your code and follow the standards.
By the way, this one fixes some -Wstrict-aliasing warnings.
port.h: Make WRITE_WORD a real function instead of a macro
port.h: Make READ_3WORD a real function instead of a macro
port.h: Make WRITE_3WORD a real function instead of a macro
port.h: Make READ_DWORD a real function instead of a macro
port.h: Make WRITE_DWORD a real function instead of a macro