Add support for building shared/dynamic libraries

[njansson]

Refactor build system to enable shared/dynamic libraries. This will ease with interfacing to python/Julia and to some extent linking with various external libraries

This is a breaking change, introducing the following

• libtool is requirement to build Neko
• Dependencies change from .o -> .lo
• shared libraries (if requested) are named libneko-${PACKAGE_VERSION}.so
• old build trees must regenerate their toolchains (./regen.sh)

Note: the default behaviour is to only build a static libneko.a (as before), shared libraries must be requested via --enable-shared

[timofeymukha]

Appears to work! I updated gitignore for the .lo and some other new stuff.