stages (build-tools, cross-tools, etc) of the build, so we can detect in
bsd.*.mk whether to pass compiler-specific flags to ${CC}.
In particular, this commit will allow using WITH_CLANG_IS_CC when the
base compiler is still gcc, and when ${CC}, ${CXX} and ${CPP} are left
at their defaults. The early stages will then be built using gcc, and
no clang-specific flags will be passed to it. The later stages will be
built as usual.
The EARLY_BUILD define can also serve other uses, such as building the
world stage C++ executables with libc++ instead of libstdc++: during the
early build stages, we cannot assume libc++ is already available, so we
must still build with libstdc++ at that time.
MFC after: 1 week
883e0f87a4