Before the build system used something like
echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(LINK) -xc - -o /dev/null -lc -Wall -Wextra -pedantic <FLAGS_TO_TEST>
to check for flags supported by the compiler (used as linker frontend).
This however no longer works with newlib 4.3.0, which requires the
symbols `_sbrk_r`, `_close_r`, `_lseek_r`, `_read_r`, and `_write_r` to
be present. Instead, now a new file `minimal_linkable.c` was added that
adds all the missing symbols and is used in the linker feature tests
instead.
