warning: argument unused during compilation: '-stdlib=libc++'

CRAN has requested the following warning be fixed (this is essentially duplicate of https://github.com/rstudio/sass/issues/120)

(Changes should be verified with https://mac.r-project.org/macbuilder/submit.html)

* checking whether package ‘sass’ can be installed ... [63s/63s] WARNING
Found the following significant warnings:
   clang: warning: argument unused during compilation: '-stdlib=libc++'
[-Wunused-command-line-argument]

from

clang -falign-functions=8 -g -O2 -Wall -pedantic -Wconversion
-Wno-sign-conversion -Wstrict-prototypes  -O2 -I ./include
-stdlib=libc++ -fPIC -c -o src/cencode.o src/cencode.c
clang: warning: argument unused during compilation: '-stdlib=libc++'
[-Wunused-command-line-argument]

-stdlib-libc++ is (obviously) a C++ option, and the latest Apple Clang
is objecting to its being passed to the C compiler.  You have

sass/src/libsass/Makefile:

ifeq ($(UNAME),Darwin)
         CFLAGS += -stdlib=libc++
         CXXFLAGS += -stdlib=libc++
         LDFLAGS += -stdlib=libc++
endif

where CFLAGS is plainly wrong, and the rest are unneeded for Apple
clang, and potentially wrong if someone chose to use GCC (which has been
done, e.g. by HomeBrew).

rstudio / sass

warning: argument unused during compilation: '-stdlib=libc++' #129