Fix compiling parameters and dependencies for macOS

[jacksonbenete]

I hope you don't mind the pull request.
This will only make it easier for compiling sam on macOS.

People on macOS don't have to change config.mk, as we'll be adding the dependencies automatically.
Do you think this is a good idea?

[jacksonbenete]

I'm detecting Darwin but not BSD though, since I don't have the various versions of BSD to test if the changes are still needed for compilation.
I removed the BSD comment, maybe we should keep it.

[jacksonbenete]

This will also address #120 .

On line 18 I've detected the system version:

OSVERSION := $(shell sw_vers | awk '($$1 = /ProductVersion:/){ ($$2 < 10.7) ? source = "-D_ANSI_SOURCE" : source = "-D_DARWIN_C_SOURCE"; print source }')
STANDARDS+=$(OSVERSION)

This is a bit cryptic, alternatively we can get more verbose:

OSVERSION_NEED_ANSI := $(shell sw_vers | awk '($$1 = /ProductVersion:/){ ($$2 < 10.7) ? ansi = 1 : ansi = 0; print ansi }')        

ifeq ($(OSVERSION_NEED_ANSI), 1) 
  STANDARDS+=-D_ANSI_SOURCE
else
  STANDARDS+=-D_DARWIN_C_SOURCE                                                                                          
endif