When configuring with --enable-fuzzing, we overwrote the CFLAGS
added by all the preceding checks. Instead of overwriting CFLAGS, append
the ones we need.

Fixes: adc28027 ("fuzz: add configure flag --enable-fuzzing")
Reported-by: Li Qiang <liq3ea@163.com>
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20200708200104.21978-2-alxndr@bu.edu>
Tested-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>