compilation error because of deprecated BIGNUM functions in openssl

[WeiWang1205]

compilation is failing on Ubuntu 20.04, the error messages are:
file included from /home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:3:
/home/w/s2sphere/tests/s2-geometry/geometry/./util/math/exactfloat/exactfloat.h: In constructor ‘ExactFloat::ExactFloat()’:
/home/w/s2sphere/tests/s2-geometry/geometry/./util/math/exactfloat/exactfloat.h:563:3: error: ‘BN_init’ was not declared in this scope
563 | BN_init(&bn_);
| ^~~~~~~
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc: In function ‘int BN_ext_count_low_zero_bits(const BIGNUM*)’:
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:75:25: error: invalid use of incomplete type ‘const BIGNUM’ {aka ‘const struct bignum_st’}
75 | for (int i = 0; i < bn->top; ++i) {
| ^~
In file included from /usr/include/openssl/bn.h:19,
from /home/w/s2sphere/tests/s2-geometry/geometry/./util/math/exactfloat/exactfloat.h:109,
from /home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:3:
/usr/include/openssl/ossl_typ.h:80:16: note: forward declaration of ‘BIGNUM’ {aka ‘struct bignum_st’}
80 | typedef struct bignum_st BIGNUM;
| ^~~~~~~~~
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:76:20: error: invalid use of incomplete type ‘const BIGNUM’ {aka ‘const struct bignum_st’}
76 | BN_ULONG w = bn->d[i];
| ^~
In file included from /usr/include/openssl/bn.h:19,
from /home/w/s2sphere/tests/s2-geometry/geometry/./util/math/exactfloat/exactfloat.h:109,
from /home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:3:
/usr/include/openssl/ossl_typ.h:80:16: note: forward declaration of ‘BIGNUM’ {aka ‘struct bignum_st’}
80 | typedef struct bignum_st BIGNUM;
| ^~~~~~~~~
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc: In constructor ‘ExactFloat::ExactFloat(double)’:
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:90:3: error: ‘BN_init’ was not declared in this scope
90 | BN_init(&bn_);
| ^~~~~~~
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc: In constructor ‘ExactFloat::ExactFloat(int)’:
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:113:3: error: ‘BN_init’ was not declared in this scope
113 | BN_init(&bn_);
| ^~~~~~~
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc: In copy constructor ‘ExactFloat::ExactFloat(const ExactFloat&)’:
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:125:3: error: ‘BN_init’ was not declared in this scope
125 | BN_init(&bn_);
make[2]: *** [CMakeFiles/s2util.dir/build.make:206: CMakeFiles/s2util.dir/util/math/exactfloat/exactfloat.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:135: CMakeFiles/s2util.dir/all] Error 2
make: *** [Makefile:130: all] Error 2

[WeiWang1205]

Although I try to replace BN_init using BN_new as said in google/s2geometry#5 , it still has error saying:

In file included from /home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:3:
/home/w/s2sphere/tests/s2-geometry/geometry/./util/math/exactfloat/exactfloat.h:503:10: error: field ‘bn_’ has incomplete type ‘BIGNUM’ {aka ‘bignum_st’}
503 | BIGNUM bn_;
| ^~~
In file included from /usr/include/openssl/bn.h:19,
from /home/w/s2sphere/tests/s2-geometry/geometry/./util/math/exactfloat/exactfloat.h:109,
from /home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:3:
/usr/include/openssl/ossl_typ.h:80:16: note: forward declaration of ‘BIGNUM’ {aka ‘struct bignum_st’}
80 | typedef struct bignum_st BIGNUM;
| ^~~~~~~~~
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc: In function ‘int BN_ext_count_low_zero_bits(const BIGNUM*)’:
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:75:25: error: invalid use of incomplete type ‘const BIGNUM’ {aka ‘const struct bignum_st’}
75 | for (int i = 0; i < bn->top; ++i) {
| ^~
In file included from /usr/include/openssl/bn.h:19,
from /home/w/s2sphere/tests/s2-geometry/geometry/./util/math/exactfloat/exactfloat.h:109,
from /home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:3:
/usr/include/openssl/ossl_typ.h:80:16: note: forward declaration of ‘BIGNUM’ {aka ‘struct bignum_st’}
80 | typedef struct bignum_st BIGNUM;
| ^~~~~~~~~
/home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:76:20: error: invalid use of incomplete type ‘const BIGNUM’ {aka ‘const struct bignum_st’}
76 | BN_ULONG w = bn->d[i];
| ^~
In file included from /usr/include/openssl/bn.h:19,
from /home/w/s2sphere/tests/s2-geometry/geometry/./util/math/exactfloat/exactfloat.h:109,
from /home/w/s2sphere/tests/s2-geometry/geometry/util/math/exactfloat/exactfloat.cc:3:
/usr/include/openssl/ossl_typ.h:80:16: note: forward declaration of ‘BIGNUM’ {aka ‘struct bignum_st’}
80 | typedef struct bignum_st BIGNUM;
| ^~~~~~~~~
make[2]: *** [CMakeFiles/s2util.dir/build.make:206: CMakeFiles/s2util.dir/util/math/exactfloat/exactfloat.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:135: CMakeFiles/s2util.dir/all] Error 2
make: *** [Makefile:130: all] Error 2