Use uname -s to select Linux vs macOS build flags instead of
hardcoding. Add -undefined dynamic_lookup for test shared libs
on macOS. Set _port=true automatically on non-Linux in ./all.
'luaD_growstack' already checks that. This commit also fixes an
internal bug in 'luaD_growstack': a large 'n' could cause an arithmetic
overflow when computing 'needed'.
The script 'all', to run all tests, automatically ensures that the
Lua interpreter and the test C libraries (in 'testes/libs/') are
updated with any changes in 'luaconf.h'.
