Makefile fix for cross-compiling from Linux.

Added a new variable TARGETSYSTEM, which is set to WINDOWS or LINUX according to
the desired system.

Calling make without parameters automatically sets the target to Linux.

Added a section for global settings for Windows and Linux.
Linux only needs -ncurses.
Windows needs additional stuff.

Relaxed the cross-compile target. The cross-compiler prefix now only needs to
contain mingw32, as there are many different versions out there.

Fixed a typo, where W32LDFLAGS was set, but W32FLAGS was used when calling the compiler.