add functionality to Makefile to build static+shared library versions

[FreshXOpenSource]

To use the seaduk version in external products it is needed, that core functions are build as libraries. The patched Makefile can now (depending on flags)

• create libduv.a (static build)
• create libduv.so, libseaduk.so (shared build)
• build nucleus as a static or shared build
• install target for binaries + libraries + headers (for external projects)
• init-duktape, init-libuv make targets for git submodule initialization
• use libuv from git submodule, deployed by the system or pkg-config

To control the buildprocess you can set in the Makefile :

LIBUV=deps/libuv (default)

LIBUV=system

LIBUV=pkg-config

BUILDTYPE=static (default)

BUILDTYPE=shared

Tested on Redhat EL7 (gcc), Darwin (clang)