Clean up the Makefile, and fix a linking problem

Use the implicit rule for single source file programs.

Specify CFLAGS and LDLIBS appropriately.

Also link against libX11 explicitly in order to fix this linking error:

  cc -ansi -Wall -pedantic -ggdb     xicursorset.c  -lXi -lXcursor  -o xicursorset
  /usr/bin/ld: /tmp/cc6cA5Ex.o: undefined reference to symbol 'XFlush'
  //usr/lib/x86_64-linux-gnu/libX11.so.6: error adding symbols: DSO missing from command line
  collect2: error: ld returned 1 exit status
  <builtin>: recipe for target 'xicursorset' failed
  make: *** [xicursorset] Error 1