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

new file mode 100644
--- /dev/null
+++ b/m4/gc-sha512.m4
@@ -0,0 +1,10 @@
+# gc-sha512.m4 serial 1
+dnl Copyright (C) 2005, 2007, 2009-2013 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_GC_SHA512],
+[
+  AC_REQUIRE([gl_GC])
+])