Since 24.05 (I think) we need to pass `-c` or Clang thinks we want to
compile *both* a final executable and precompiled header file, and
complains that we cannot use `-o` with multiple outputs. `-c` seems fine
with GCC too, so I just put it in there conditionally.
In cross, CXX will look like aarch64-unknown-linux-gnu-g++. We could
run some command to check what kind of compiler it is, but for now we
can just check if g++ is anywhere in the string. I couldn’t find any
"ends with" for makefile, so it can be anywhere in CXX.
