CCFLAGS => '-Wall -g -O2 --std=gnu99',

##</code><code>##

$ $ perl -MConfig -E 'say $Config{ccflags}'
-fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2

##</code><code>##

use Config;

...
   CCFLAGS => "--std=gnu99 $Config{ccflags}", # if that works, add other stuff, like -g and -Wall...
...