Fix Makefile.Unix

<code>
gcc -c -o file-util.o file-util.c -I/usr/X11R6/include
gcc -c -o gl-util.o gl-util.c -I/usr/X11R6/include
gcc -c -o meshes.o meshes.c -I/usr/X11R6/include
gcc -c -o flag.o flag.c -I/usr/X11R6/include
gcc -o flag file-util.o gl-util.o meshes.o flag.o -L/usr/X11R6/lib -lGL -lglut -lGLEW
/usr/bin/ld: meshes.o: undefined reference to symbol 'cosf@@GLIBC_2.2.5'
/usr/bin/ld: note: 'cosf@@GLIBC_2.2.5' is defined in DSO /usr/lib/libm.so.6 so try adding it to the linker command line
/usr/lib/libm.so.6: could not read symbols: Invalid operation
collect2: error: ld returned 1 exit status
make: *** [flag] Error 1
</code>

Running make on Arch Linux delivered the above output. I just added -lm to the linker flags, and renamed Nmakefile.Windows to Makefile.Windows.