configure: Also display the library dir in the summary

configure: Also display the library dir in the summary

The summary now looks like:

Window Maker was configured as follows:

Installation path prefix            : /usr/local
Installation path for binaries      : /usr/local/bin
Installation path for libraries     : /usr/lib64
Installation path for WPrefs.app    : /usr/local
Supported graphic format libraries  : XPM PNG JPEG TIFF builtin-PPM
Antialiased text support in WINGs   : yes
Xinerama extension support          : yes
XRandR extension support            : yes
Translated message files to install : None

I want to see the library line in order to avoid forgetting that
I should put them in /usr/lib64 (and not in the defaul /usr/local/lib)