configure.ac: Check C99 compiler

configure.ac: Check C99 compiler

This ends up to add -std=gnu99 and fixes the following
compilation problem introduced by commit 08feeb75.
("lib/flow: Use C99 declaration in for statement.")

libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I.. -I ../include -I ../lib -I ./lib -Wstrict-prototypes -Wall -Wextra -Wno-sign-compare -Wpointer-arith -Wno-format-zero-length -Wswitch-enum -Wunused-parameter -Wstrict-aliasing -Wbad-function-cast -Wcast-align -Wmissing-prototypes -Wmissing-field-initializers -g -O2 -MT lib/classifier.lo -MD -MP -MF lib/.deps/classifier.Tpo -c ../lib/classifier.c -o lib/classifier.o
../lib/classifier.c: In function 'miniflow_and_mask_matches_flow':
../lib/classifier.c:1722:5: error: 'for' loop initial declarations are only allowed in C99 mode
../lib/classifier.c:1722:5: note: use option -std=c99 or -std=gnu99 to compile your code
Makefile:3013: recipe for target 'lib/classifier.lo' failed

% gcc --version
gcc (NetBSD nb1 20120916) 4.5.4
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>