"ALL_BUILD_CFLAGS" and "native-compile" in Makerules

[Roland McGrath]

> The reason I noticed this is that a "make check" will fail to compile
> elf/check-textrel.c if the host system doesn't have elf.h on it. Obviously,
> this scenario is not very common, but it has come up a few times lately I've
> noticed.

check-textrel.c is not portable to systems like that.  It is built on the
build system, but assumes the build system has some GNU-like libc interfaces.