switching to clang in base.
add COMPILER_LIBCXX where c++abi is used. there will be some "Extra"s but
this is the simplest way to use c++abi on base=clang and estdc++ on base=gcc
arches.
MODGNUSTEP_NEEDS_BASE set to yes, to add it to link
depends, even if it not directly links against -base,
gnustep libraries/frameworks assume that applications
link against -base when using those libs/frameworks.
build problems pointed out by rpe@