examples/Makefile: disable -O2 optimizations

There is no much point in optimizing example code, and the -O2 flag is
annoying when debugging.

doc/examples/Makefile

@@ -7,7 +7,7 @@ FFMPEG_LIBS=    libavdevice                        \
                 libswscale                         \
                 libavutil                          \
 
-CFLAGS += -Wall -O2 -g
+CFLAGS += -Wall -g
 CFLAGS := $(shell pkg-config --cflags $(FFMPEG_LIBS)) $(CFLAGS)
 LDLIBS := $(shell pkg-config --libs $(FFMPEG_LIBS)) $(LDLIBS)