Auto-detect platform in makefiles and test runner

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.

testes/libs/makefile

@@ -6,6 +6,10 @@ CC = gcc
 
 # compilation should generate Dynamic-Link Libraries
 CFLAGS = -Wall -O2 -I$(LUA_DIR) -fPIC -shared
+UNAME = $(shell uname -s)
+ifeq ($(UNAME),Darwin)
+CFLAGS += -undefined dynamic_lookup
+endif
 
 # libraries used by the tests
 all: lib1.so lib11.so lib2.so lib21.so lib2-v2.so