build: avoid -lgcrypt with newer gnutls

https://bugzilla.redhat.com/show_bug.cgi?id=951637

Newer gnutls uses nettle, rather than gcrypt, which is a lot nicer
regarding initialization.  Yet we were unconditionally initializing
gcrypt even when gnutls wouldn't be using it, and having two crypto
libraries linked into libvirt.so is pointless, but mostly harmless
(it doesn't crash, but does interfere with certification efforts).

There are three distinct version ranges to worry about when
determining which crypto lib gnutls uses, per these gnutls mails:
2.12: http://lists.gnu.org/archive/html/gnutls-devel/2011-03/msg00034.html
3.0: http://lists.gnu.org/archive/html/gnutls-devel/2011-07/msg00035.html

If pkg-config can prove version numbers and/or list the crypto
library used for static linking, we have our proof; if not, it
is safer (even if pointless) to continue to use gcrypt ourselves.

* configure.ac (WITH_GNUTLS): Probe whether to add -lgcrypt, and
define a witness WITH_GNUTLS_GCRYPT.
* src/libvirt.c (virTLSMutexInit, virTLSMutexDestroy)
(virTLSMutexLock, virTLSMutexUnlock, virTLSThreadImpl)
(virGlobalInit): Honor the witness.
* libvirt.spec.in (BuildRequires): Make gcrypt usage conditional,
no longer needed in Fedora 19.

Signed-off-by: Eric Blake <eblake@redhat.com>

configure.ac

@@ -1086,12 +1086,26 @@ if test "x$with_gnutls" != "xno"; then
   LIBS="$LIBS $GNUTLS_LIBS"
 
   GNUTLS_FOUND=no
+  GNUTLS_GCRYPT=unknown
   if test -x "$PKG_CONFIG" ; then
+    dnl Triple probe: gnutls < 2.12 only used gcrypt, gnutls >= 3.0 uses
+    dnl only nettle, and versions in between had a configure option.
+    dnl Our goal is to avoid gcrypt if we can prove gnutls uses nettle,
+    dnl but it is a safe fallback to use gcrypt if we can't prove anything.
+    if $PKG_CONFIG --exists 'gnutls >= 3.0'; then
+      GNUTLS_GCRYPT=no
+    elif $PKG_CONFIG --exists 'gnutls >= 2.12'; then
+      GNUTLS_GCRYPT=probe
+    else
+      GNUTLS_GCRYPT=yes
+    fi
     PKG_CHECK_MODULES(GNUTLS, gnutls >= $GNUTLS_REQUIRED,
       [GNUTLS_FOUND=yes], [GNUTLS_FOUND=no])
   fi
   if test "$GNUTLS_FOUND" = "no"; then
+    dnl pkg-config couldn't help us, assume gcrypt is necessary
     fail=0
+    GNUTLS_GCRYPT=yes
     AC_CHECK_HEADER([gnutls/gnutls.h], [], [fail=1])
     AC_CHECK_LIB([gnutls], [gnutls_handshake],[], [fail=1], [-lgcrypt])
 
@@ -1108,13 +1122,22 @@ if test "x$with_gnutls" != "xno"; then
       AC_MSG_ERROR([You must install the GnuTLS library in order to compile and run libvirt])
     fi
   else
-    dnl Not all versions of gnutls include -lgcrypt, and so we add
-    dnl it explicitly for the calls to gcry_control/check_version
-    GNUTLS_LIBS="$GNUTLS_LIBS -lgcrypt"
-
-    dnl We're not using gcrypt deprecated features so define
-    dnl GCRYPT_NO_DEPRECATED to avoid deprecated warnings
-    GNUTLS_CFLAGS="$GNUTLS_CFLAGS -DGCRYPT_NO_DEPRECATED"
+    dnl See comments above about when to use gcrypt.
+    if test "$GNUTLS_GCRYPT" = probe; then
+      case `$PKG_CONFIG --libs --static gnutls` in
+        *gcrypt*) GNUTLS_GCRYPT=yes     ;;
+        *nettle*) GNUTLS_GCRYPT=no      ;;
+        *)        GNUTLS_GCRYPT=unknown ;;
+      esac
+    fi
+    if test "$GNUTLS_GCRYPT" = yes || test "$GNUTLS_GCRYPT" = unknown; then
+      GNUTLS_LIBS="$GNUTLS_LIBS -lgcrypt"
+      dnl We're not using gcrypt deprecated features so define
+      dnl GCRYPT_NO_DEPRECATED to avoid deprecated warnings
+      GNUTLS_CFLAGS="$GNUTLS_CFLAGS -DGCRYPT_NO_DEPRECATED"
+      AC_DEFINE_UNQUOTED([WITH_GNUTLS_GCRYPT], 1,
+        [set to 1 if it is known or assumed that GNUTLS uses gcrypt])
+    fi
 
     dnl gnutls 3.x moved some declarations to a new header
     AC_CHECK_HEADERS([gnutls/crypto.h], [], [], [[

libvirt.spec.in

@@ -438,7 +438,9 @@ BuildRequires: readline-devel
 BuildRequires: ncurses-devel
 BuildRequires: gettext
 BuildRequires: libtasn1-devel
+%if (0%{?rhel} && 0%{?rhel} < 7) || (0%{?fedora} && 0%{?fedora} < 19)
 BuildRequires: libgcrypt-devel
+%endif
 BuildRequires: gnutls-devel
 BuildRequires: libattr-devel
 %if %{with_libvirtd}

src/libvirt.c

@@ -55,7 +55,9 @@
 #include "intprops.h"
 #include "virconf.h"
 #if WITH_GNUTLS
-# include <gcrypt.h>
+# if WITH_GNUTLS_GCRYPT
+#  include <gcrypt.h>
+# endif
 # include "rpc/virnettlscontext.h"
 #endif
 #include "vircommand.h"
@@ -270,7 +272,7 @@ winsock_init(void)
 #endif
 
 
-#ifdef WITH_GNUTLS
+#ifdef WITH_GNUTLS_GCRYPT
 static int virTLSMutexInit(void **priv)
 {
     virMutexPtr lock = NULL;
@@ -323,7 +325,7 @@ static struct gcry_thread_cbs virTLSThreadImpl = {
     virTLSMutexUnlock,
     NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL
 };
-#endif
+#endif /* WITH_GNUTLS_GCRYPT */
 
 /* Helper macros to implement VIR_DOMAIN_DEBUG using just C99.  This
  * assumes you pass fewer than 15 arguments to VIR_DOMAIN_DEBUG, but
@@ -407,7 +409,7 @@ virGlobalInit(void)
         virErrorInitialize() < 0)
         goto error;
 
-#ifdef WITH_GNUTLS
+#ifdef WITH_GNUTLS_GCRYPT
     /*
      * This sequence of API calls it copied exactly from
      * gnutls 2.12.23 source lib/gcrypt/init.c, with