Add conditionals to build a clang-minimal package
clang-minimal is a package that contains only the compiler binaries
(clang, clang++, and clang-cpp) and is statically linked against LLVM
so it has no runtime dependency on llvm-libs. This package is meant
to be used for building a newer version of clang in a buildroot with
clang as the default compiler. Normally, clang is unable to build a
newer version of itself, because the two versions depend on different
versions of llvm-libs which cannot both be installed at the same time.