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.
The symlink of the shared library in lib doesn't point to
GNUstep/Frameworks/$FRAMEWORK.framework/Versions/Current/libnetclasses.so.${SHLIB_VERSION}
anymore but now to
GNUstep/Frameworks/netclasses.framework/Versions/${SHLIB_VERSION}/libnetclasses.so.${SHLIB_VERSION}
It's not easily visible in pkg/PLIST, but on package creation plistdb catched it
problem pointed out by nigel@
