bug: extension for dynamic libraries is incorrect in mac

[richard-ramos]

https://github.com/waku-org/nwaku/blob/9808e205af522989cc6888c12e6656df47e0da48/Makefile#L427-L435

In mac it should be .dylib... maybe something like this should do the trick (copied from status-go makefile)

ifeq ($(OS),Windows_NT)     # is Windows_NT on XP, 2000, 7, Vista, 10...
 	detected_OS := Windows
else
 	detected_OS := $(strip $(shell uname))
endif

ifeq ($(detected_OS),Darwin)
 	LIBWAKU_EXT := dylib
else ifeq ($(detected_OS),Windows)
 	LIBWAKU_EXT := dll
else
 	LIBWAKU_EXT := so
endif

ifeq ($(STATIC), 1) 
	echo -e $(BUILD_MSG) "build/$@.a" && $(ENV_SCRIPT) nim libwakuStatic $(NIM_PARAMS) waku.nims 
else
	echo -e $(BUILD_MSG) "build/$@.$(LIBWAKU_EXT)" && $(ENV_SCRIPT) nim libwakuDynamic $(NIM_PARAMS) waku.nims 
endif