gc: support HMAC-SHA256 and HMAC-SHA512.

gc: support HMAC-SHA256 and HMAC-SHA512.

* lib/gc.h: Add gc_hmac_sha256 and gc_hmac_sha512.
* lib/gc-libgcrypt.c (gc_hmac_sha256, gc_hmac_sha512): New
functions.
(gc_hmac_md5): Use symbolic constant.
* lib/gc-gnulib.c: Include hmac.h for HMAC-SHA256/512 too.
(gc_hmac_sha256, gc_hmac_sha512): New functions.
* lib/hmac.h: Add hmac_sha256 and hmac_sha512 prototypes.
* m4/sha256.m4: Protect against empty expansion.
* m4/sha512.m4: Likewise.
* lib/hmac-sha256.c: New file.
* lib/hmac-sha512.c: Likewise.
* m4/gc-hmac-sha256.m4: Likewise.
* m4/gc-hmac-sha512.m4: Likewise.
* m4/gc-sha256.m4: Likewise.
* m4/gc-sha512.m4: Likewise.
* modules/crypto/gc-hmac-sha256: Likewise.
* modules/crypto/gc-hmac-sha256-tests: Likewise.
* modules/crypto/gc-hmac-sha512: Likewise.
* modules/crypto/gc-hmac-sha512-tests: Likewise.
* modules/crypto/hmac-sha256: Likewise.
* modules/crypto/hmac-sha256-tests: Likewise.
* modules/crypto/hmac-sha512: Likewise.
* modules/crypto/hmac-sha512-tests: Likewise.
* tests/test-gc-hmac-sha256.c: Likewise.
* tests/test-gc-hmac-sha512.c: Likewise
* tests/test-hmac-sha256.c: Likewise.
* tests/test-hmac-sha512.c: Likewise