libcxx on older systems (eg 10.6) test and fixes format

[Ken Cunningham]

Hello,

I've been submitting portfile fixes for the above system as I come across them.

I've submitted fixes for qt4-mac, aria2, and lilypond so far.

But each has a slightly different test format, and there are different opinions about how to do it, so I'd like to standardize that test now.

I propose for a specific snowleopard with libcxx fix, use this:

# Snow Leopard with cxx_stdlib=libc++ fix
platform darwin { 
	if {{${MINOR} == 6} && {${configure.cxx_stdlib} eq "libc++"}} { 

			#do stuff like configure.ldflags-append    "-lc++" 

	} 
} 


and for 10.9 or lower with libcxx, use this:

# Mountain Lion or older with cxx_stdlib=libc++ fix
platform darwin { 
	if {{${MINOR} <= 9} && {${configure.cxx_stdlib} eq "libc++"}} { 

			#do stuff like configure.ldflags-append    "-lc++" 

	} 
} 


To change from one to the other requires changing only two characters in the MINOR test, and it seems suitably specific, concise, and clear.

Feedback or thoughts?

Ken