examples/Makefile.am: also include global flags to compile mandelbrot

examples/Makefile.am

@@ -678,6 +678,6 @@ mandelbrot_mandelbrot_SOURCES =			\
 	mandelbrot/mandelbrot.c
 
 if HAVE_X11
-mandelbrot_mandelbrot_CPPFLAGS = $(X_CFLAGS)
+mandelbrot_mandelbrot_CPPFLAGS = $(X_CFLAGS) $(AM_CFLAGS) $(AM_CPPFLAGS)
 mandelbrot_mandelbrot_LDADD = $(X_PRE_LIBS) -lX11 $(X_LIBS) $(X_EXTRA_LIBS)
 endif