build: add apisupport low_level_rsa

OpenSSL 3.0 marked many RSA functions as "deprecated"; tarsnap uses some
of them[1] in:
    lib/crypto/crypto_compat.c
    lib/crypto/crypto_keys.c
    lib/crypto/crypto_keys_subr.c
    lib/crypto/crypto_rsa.c

Those files are now being compiled into a new libtarsnap_rsa.a, which
uses the appropriate compiler flags.

[1] List of relevant deprecated OpenSSL 3.0 RSA functions:
    RSAPublicKey_dup
    RSA_bits
    RSA_free
    RSA_generate_key_ex
    RSA_get0_crt_params
    RSA_get0_factors
    RSA_get0_key
    RSA_new
    RSA_private_decrypt
    RSA_private_encrypt
    RSA_public_decrypt
    RSA_public_encrypt
    RSA_set0_crt_params
    RSA_set0_factors
    RSA_set0_key
    RSA_size

Makefile.am

@@ -14,6 +14,7 @@ noinst_LIBRARIES=							\
 	lib/libcperciva_rdrand.a					\
 	lib/libcperciva_shani.a						\
 	lib/libtarsnap.a						\
+	lib/libtarsnap_rsa.a						\
 	lib/libtarsnap_sse2.a						\
 	libarchive/libarchive.a
 bin_PROGRAMS=								\
@@ -124,17 +125,13 @@ lib_libtarsnap_a_SOURCES=						\
 	lib-platform/util/ts_getfstype.c				\
 	lib-platform/util/ts_getfstype.h				\
 	lib/crypto/crypto.h						\
-	lib/crypto/crypto_compat.c					\
 	lib/crypto/crypto_compat.h					\
 	lib/crypto/crypto_file.c					\
 	lib/crypto/crypto_hash.c					\
 	lib/crypto/crypto_internal.h					\
-	lib/crypto/crypto_keys.c					\
 	lib/crypto/crypto_keys_init.c					\
 	lib/crypto/crypto_keys_server.c					\
-	lib/crypto/crypto_keys_subr.c					\
 	lib/crypto/crypto_passwd_to_dh.c				\
-	lib/crypto/crypto_rsa.c						\
 	lib/crypto/crypto_scrypt_smix.c					\
 	lib/crypto/crypto_scrypt_smix.h					\
 	lib/crypto/crypto_scrypt_smix_sse2.h				\
@@ -422,6 +419,15 @@ nodist_lib_libcperciva_cpusupport_detect_a_SOURCES=			\
 lib_libcperciva_cpusupport_detect_a_CPPFLAGS=$(lib_libtarsnap_a_CPPFLAGS)
 LIBTARSNAP_A+=	lib/libcperciva_cpusupport_detect.a
 
+lib_libtarsnap_rsa_a_SOURCES=						\
+	lib/crypto/crypto_compat.c					\
+	lib/crypto/crypto_keys.c					\
+	lib/crypto/crypto_keys_subr.c					\
+	lib/crypto/crypto_rsa.c
+lib_libtarsnap_rsa_a_CPPFLAGS=$(lib_libtarsnap_a_CPPFLAGS)
+lib_libtarsnap_rsa_a_CFLAGS=`. ./apisupport-config.h; echo $${CFLAGS_LIBCRYPTO_LOW_LEVEL_RSA}`
+LIBTARSNAP_A+=	lib/libtarsnap_rsa.a
+
 
 #
 #

libcperciva/apisupport/Build/apisupport-LIBCRYPTO-LOW_LEVEL_RSA.c

@@ -0,0 +1,14 @@
+#include <openssl/rsa.h>
+
+int
+main(void)
+{
+	RSA * rsa;
+
+	/* Allocate and free a RSA key. */
+	rsa = RSA_new();
+	RSA_free(rsa);
+
+	/* Success! */
+	return (0);
+}

libcperciva/apisupport/Build/apisupport.sh

@@ -83,3 +83,5 @@ feature LIBSSL HOST_NAME "-lssl" ""			\
 	"-Wno-cast-qual"
 feature LIBCRYPTO LOW_LEVEL_AES "-lcrypto" ""		\
 	"-Wno-deprecated-declarations"
+feature LIBCRYPTO LOW_LEVEL_RSA "-lcrypto" ""		\
+	"-Wno-deprecated-declarations"