vanilla 2.4.20 with freeswan-2.0-rc1 fails to build

crypto.o(.text+0x16): In function `init_crypto':
/usr/src/freeswan-2.00-rc1/programs/pluto/crypto.c:50: undefined reference to `mpz_init_set_str'
crypto.o(.text+0x31):/usr/src/freeswan-2.00-rc1/programs/pluto/crypto.c:50: undefined reference to `mpz_init_set_str'
crypto.o(.text+0x4c):/usr/src/freeswan-2.00-rc1/programs/pluto/crypto.c:50: undefined reference to `mpz_init_set_str'
defs.o(.text+0x69): In function `mpz_to_n':

At the time, we seemed to believe this was a missing 'make mrproper' in
the redhat source tree, but this is using a linus 2.4.20 tree, with
symlinks in /usr/include/asm|linux pointing to /usr/src/linux/include

I used:

zlib decompression support (CONFIG_ZLIB_INFLATE) [M/n/y/?] 
zlib compression support (CONFIG_ZLIB_DEFLATE) [M/n/y/?] 

The system is Redhat 8.0

Paul