Include the executable suffix within make

This makes sure that e.g. "make clean" actually removes
the built exe files, and avoids relinking the exe files each
time make is run when cross compiling from linux to windows.

(Make on windows seems to have an exception that knows to skip
rebuilding the 'h264enc' target if 'h264enc' doesn't exist but
'h264enc.exe' does, but this exception doesn't work on normal
unix make.)
Merge pull request #199 from mstorsjo/make-exeext

Include the executable suffix within make