Fix SConstruct for darwin.

[schulze]

This fixes the build to respect CC=gcc on darwin and to ignore a
warning that breaks the build with xcodes's clang on my system.

[prashantbarca]

@schulze -Wno-unknown-pragmas sufficed for me to get it to work with clang!

[schulze]

@prashantbarca -Wno-unknown-pragmas does not suffice on this system:

clang -o build/opt/src/backends/regex.os -c -std=gnu99 -Wall -Wextra -Werror -Wno-unused-parameter -Wno-attributes -Wno-unused-variable -Wno-unknown-pragmas -O3 -fPIC 
build/opt/src/backends/regex.c
build/opt/src/backends/regex.c:229:32: error: unknown warning group '-Wclobbered', ignored
  [-Werror,-Wunknown-warning-option]
#pragma GCC diagnostic ignored "-Wclobbered"
                           ^
1 error generated.

Mac OS X is not my native habitat, so it might well be that I am missing something here. Xcode should be up to date.

[schulze]

As the error message say, the problem here is not a unknown pragma, but the unknown warning that is introduced by the pragma.