BUG #16379: configure with krb5 in centos6 will compile without linking to library com_err and krb5

The following bug has been logged on the website:

Bug reference: 16379
Logged by: Shaoqi Bai
Email address: sbai(at)pivotal(dot)io
PostgreSQL version: Unsupported/Unknown
Operating system: Linux
Description:

postgres version is using branch REL9_0_23

docker pull centos:6
docker run --rm -v ~/postgres:/tmp/b centos:6
yum install -y krb5-devel krb5-libs krb5-server gcc
./configure -v --with-krb5 --without-readline --without-zlib
the output is like following
```
checking for library containing com_err... -lkrb5
checking for library containing krb5_sendauth... none required
```
docker pull centos:7
docker run --rm -v ~/postgres:/tmp/b centos:7
yum install -y krb5-devel krb5-libs krb5-server gcc
./configure -v --with-krb5 --without-readline --without-zlib
the output is like following
```
checking for library containing com_err... -lcom_err
checking for library containing krb5_sendauth... -lkrb5
```