Currently, rustc defaults to invoking `cc`, even if `HOSTCC` is defined,Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>
resulting in build failures in hermetic environments where `cc` does not
exist. This includes both hostprogs and proc-macros.
Since we are setting the linker to `HOSTCC`, we set the linker flavor to
`gcc` explicitly. The linker-flavor selects both which linker to search
for if the linker is unset, and which kind of linker flags to pass.
Without this flag, `rustc` would attempt to determine which flags to
pass based on the name of the binary passed as `HOSTCC`. `gcc` is the
name of the linker-flavor used by `rustc` for all C compilers, including
both `gcc` and `clang`.
Signed-off-by: Matthew Maurer <mmaurer@xxxxxxxxxx>
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