Fix errors due to using CFLAGS -Wall -Werror

vmod_querystring.c: In function 'apply_filter':
vmod_querystring.c:301:14: error: unused variable 'params' [-Werror=unused-variable]
Move local CFLAGS to AM_CFLAGS
Merge pull request #8 from zmousm/grnet/master

fix warning treated as error, caused by using CFLAGS -Wall -Werror