Compiling the rpi-3.18.9-rt5 kernel

I’m following the instruction in GitHub, but I’m getting the error below. Do I need to set another another env variable beside the ARCH and CROSS_COMPILE?

[abee: linux-rt-rpi]$ make -j5
scripts/kconfig/conf --silentoldconfig Kconfig
CHK include/config/kernel.release
CHK include/generated/uapi/linux/version.h
CHK include/generated/utsrelease.h
HOSTCC scripts/dtc/dtc.o
HOSTCC scripts/genksyms/genksyms.o
HOSTCC scripts/dtc/flattree.o
CC scripts/mod/empty.o
HOSTCC scripts/mod/mk_elfconfig
HOSTCC scripts/genksyms/
scripts/mod/mk_elfconfig.c:4:10: fatal error: ‘elf.h’ file not found
#include <elf.h>
1 error generated.
make[2]: *** [scripts/mod/mk_elfconfig] Error 1
make[1]: *** [scripts/mod] Error 2
make[1]: *** Waiting for unfinished jobs…
HOSTCC scripts/dtc/fstree.o
HOSTCC scripts/genksyms/lex.lex.o
HOSTCC scripts/dtc/data.o
HOSTCC scripts/dtc/livetree.o
HOSTCC scripts/dtc/treesource.o
HOSTLD scripts/genksyms/genksyms
HOSTCC scripts/dtc/srcpos.o
HOSTCC scripts/dtc/checks.o
HOSTCC scripts/dtc/util.o
HOSTCC scripts/dtc/dtc-lexer.lex.o
HOSTCC scripts/dtc/
HOSTLD scripts/dtc/dtc
make: *** [scripts] Error 2
make: *** Waiting for unfinished jobs…