Makefile: Append EXEEXT variable to executable filename rules

Makefile: Append EXEEXT variable to executable filename rules

If x86_64-w64-mingw32-gcc compiler is called with -o filename option
without any file extension then compiler automatically appends suffix
".exe" to output filename.

This behavior of x86_64-w64-mingw32-gcc compiler basically breaks pattern
rule of type '%: %.o' as x86_64-w64-mingw32-gcc compiler cannot generate
arbitrary output file via -o option just by stripping .o extension from
filename.

When generating executables by x86_64-w64-mingw32-gcc compiler it is
really the best option to specify .exe suffix in -o option.

So introduce a new makefile variable EXEEXT which will be automatically
appended to any executable filename. For Windows and DOS systems set it
to ".exe". For other systems set it just to empty string "".

GNU automake uses same makefile variable for same purpose.