clamav: fix configure breakage after zlib 1.2.10 version bump

clamav: fix configure breakage after zlib 1.2.10 version bump

Fixes:
http://autobuild.buildroot.net/results/b6b/b6ba2dfb42ee41ed0b8304aa8c78645245f3b341/
http://autobuild.buildroot.net/results/eef/eef9a2dda2c172cd600dc74c1e5e60476d92280d/
http://autobuild.buildroot.net/results/827/82798118795aa6334b4dd6eac06777682131da7f/

The clamav configure script by default checks for old zlib versions with
known vulnerabilities and errors out if found:

configure: error: The installed zlib version may contain a security bug.
Please upgrade to 1.2.2 or later: http://www.zlib.net.  You can omit this
check with --disable-zlib-vcheck but DO NOT REPORT any stability issues
then!

The check is unfortunately not very robust as it simply checks for a version
string matching '1.2.1' (which 1.2.10 does):

vuln=`grep "ZLIB_VERSION \"1.2.1" $ZLIB_HOME/include/zlib.h`

As a workaround, pass --disable-zlib-vcheck to skip this check.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>