Add support for the IDEA cipher.

Adapt idea.c to the Libgcrypt framework.
Add IDEA to cipher_table and to the build system.

Patents on IDEA have expired:

Europe: EP0482154 on 2011-05-16,
Japan:  JP3225440 on 2011-05-16,
U.S.:   5,214,703 on 2012-01-07.

• configure.ac: Add idea to the list of available ciphers.

Define USE_IDEA if idea is enabled.

• cipher/cipher.c (cipher_table): Add entry for IDEA.
• cipher/idea.c: Update comment about patents.

Include proper header files and remove redundant declarations.
(expand_key, cipher, do_setkey, encrypt_block, decrypt_block):
Define function arguments as const where appropriate.
(cipher): Test for !WORDS_BIGENDIAN instead of LITTLE_ENDIAN_HOST.
(do_setkey, decrypt_block): Don't call selftest.
(idea_setkey): New function, wrapper for do_setkey.
(idea_encrypt): New function, wrapper for encrypt_block.
(_gcry_cipher_spec_idea): Define.

• cipher/Makefile.am (EXTRA_libcipher_la_SOURCES): Add idea.c.
• src/cipher.h (_gcry_cipher_spec_idea): Declare.
• tests/basic.c (check_ciphers): Add GCRY_CIPHER_IDEA.