Make it compile on Mac OS using clang

[egrath]

To build RunVT without problems on Mac OS two things need to be changed:

1. pty.h does not exist on Mac OS but instead uses util.h - solved with preprocessor conditional
2. Garbage collection of unused sections (linker flag -Wl,--gc-sections) and stripping (linker flag -s) is not supported/deprecated on Mac OS and needs different flags - solved with a conditional in the Makefile