Makefile.sharedlibrary on OSX should use -installname instead -soname

[degyves]

On OSX High Sierra with clang and GNU Make 3.81, Apple LLVM version 9.0.0 (clang-900.0.39.2):

make will stop since -soname is unknown. On lines 49 and 52 of dist/Makefile.sharedlibrary they should be changed to -installname:

line 49: $(CC) -shared -fPIC -Wall -Wextra -Os -Wl,-install_name,libduktape.so.$(SONAME_VERSION) \

line 53: $(CC) -shared -fPIC -g -Wall -Wextra -Os -Wl,-install_name,libduktaped.so.$(SONAME_VERSION) \

[svaarala]

As background information, the Makefiles in the distributable are not intended for production use and are not generally portable to all targets Duktape itself ports to. There are simply too many targets for them to be officially supported; for instance, the sharedlibrary Makefile simply provides an example how one can roughly compile Duktape as a shared library, and the details may vary depending on the target (even for Linux targets).

That said, if there's a simple diff that will make it work on OSX and won't break it on Linux, I'll be happy to apply the diff.