shared library version should be optional

[daurnimator]

Not all shared libraries should have a version; the version parameter to addSharedLibrary in the zig build system should be optional.

[andrewrk]

Let's make it a tagged union, so that at least people are encouraged to choose to not have a version, vs that being the default.

[waruqi]

I also very much hope there is a way to remove '0.0.0' from the dynamic library file name.

on linux:

generate libtest.so.0.0.0 and lld test -> libtest.so.0 not found! oh, no..

on macOS

generate libtest.0.0.0.dylib. I also cannot link -ltest.

for example

[ 37%]: ccache compiling.release src/main.zig
/usr/local/bin/zig build-obj --release-fast --output-dir build/.objs/test/macosx/x86_64/release/src --name main.zig src/main.zig
[ 50%]: ccache compiling.release src/test.zig
/usr/local/bin/zig build-obj --release-fast -fPIC --output-dir build/.objs/testlib/macosx/x86_64/release/src --name test.zig src/test.zig
[ 62%]: linking.release libtestlib.dylib
/usr/local/bin/zig build-lib --strip -dynamic -fPIC --output-dir build/macosx/x86_64/release --name testlib --object build/.objs/testlib/macosx/x86_64/release/src/test.zig.o
[ 87%]: linking.release test
/usr/local/bin/zig build-exe -Lbuild/macosx/x86_64/release -rpath @loader_path --strip -ltestlib --output-dir build/macosx/x86_64/release --name test --object build/.objs/test/macosx/x86_64/release/src/main.zig.o
error: lld: error: Unable to find library for -ltestlib