Shared library and installation

[remicollet]

Please provide configure option to allow

• installation prefix (ex: /usr)
• include directory (ex: /usr/include/njs)
• library directory (ex: /usrs/lib64)
• shared library (on/off)
• static library (on/off)

So this can be used as all libraries
allowing unit to use it, without having to rebuild it on each version
and also reducing unit executable size (grow from ~480KB to ~1300KB with the static library)