examples/Makefile.am: fix flags for mandelbrot compilation

examples/Makefile.am

@@ -677,7 +677,8 @@ examplebin_PROGRAMS +=				\
 mandelbrot_mandelbrot_SOURCES =			\
 	mandelbrot/mandelbrot.c
 
+mandelbrot_mandelbrot_CPPFLAGS = $(AM_CFLAGS) $(AM_CPPFLAGS)
 if HAVE_X11
-mandelbrot_mandelbrot_CPPFLAGS = $(X_CFLAGS) $(AM_CFLAGS) $(AM_CPPFLAGS)
+mandelbrot_mandelbrot_CPPFLAGS += $(X_CFLAGS)
 mandelbrot_mandelbrot_LDADD = $(X_PRE_LIBS) -lX11 $(X_LIBS) $(X_EXTRA_LIBS)
 endif