configure: fixed X11 libraries detection on RedHat/CentOS

With this fix, elvis can be built from source on rhel-6.5 (and centos-6.5) on x86_64.
I've configured it like this:

```sh
$ ./configure --verbose --with-x --x-libraries=/usr/lib64 --prefix=/usr/local/opt/elvis-2.2-git
```

And it has now successfully detected `Xpm` and `Xft`:

```
   [...]
   X11 headers found
   X11 libraries found
   GCC found
   Enabling the use of background images since -lXpm and <X11/xpm.h> were found
   Xft found -- elvis will use it
   [...]
```