Add build tag and appropriate LDFLAGS for macOS support

[matiasinsaurralde]

OpenFHE provides macOS support.

After building OpenFHE on macOS I've noticed that current main branch doesn't build on macOS:

% go test . -tags 'openfhe'                                               
# github.com/collapsinghierarchy/openfhe-go.test
ld: warning: ignoring duplicate libraries: '-lc++'
dyld[48543]: Library not loaded: @rpath/libOPENFHEcore.1.dylib
  Referenced from: <A2C383E2-3C60-3420-FB90-F4D5EFC96691> /private/var/folders/s5/3_m33khx5njfpz_5g1d4wrkc0000gn/T/go-build1261414508/b001/openfhe-go.test
  Reason: no LC_RPATH's found
signal: abort trap
FAIL	github.com/collapsinghierarchy/openfhe-go	0.310s
FAIL

The change adds -Wl,-rpath,/usr/local/lib to macOS (darwin) linker flags, macOS 10.5 and above use/expect rpath for search path:

% go test . -tags 'openfhe'
# github.com/collapsinghierarchy/openfhe-go.test
ld: warning: ignoring duplicate libraries: '-lc++'
ok  	github.com/collapsinghierarchy/openfhe-go	0.328s

The introduction of these linker flags enable macOS support for openfhe-go and the usage of the darwin build tag will ensure they're only used for macOS builds 👍