Installing headers with 0.9.29 spawns gcc errors

[Paul Brook]

> -       KERNEL_HEADERS="${KERNEL_HEADERS}" top_builddir=. CC="$(CC)
> $(CPU_CFLAGS)" $(SHELL) extra/scripts/gen_bits_syscall_h.sh > $$tmp; \ +   
>    KERNEL_HEADERS="${KERNEL_HEADERS}" top_builddir=. CC=$(HOSTCC) $(SHELL)
> extra/scripts/gen_bits_syscall_h.sh > $$tmp; \

This is definitely wrong. The output of the compiler depends on the 
configuration of and options passed to the compiler.

For example, on x86-64 machines asm/unistd.h defines different sets of 
syscalls depending whether it's in 32-bit or 64-bit mode.

If you don't have a target compiler available you simply can't generate this 
header.

Paul