configure: Link test before auto-enabling crypto libraries

At least ubuntu 18.04 does not package static gnutls libraries.
At least Fedora 30 does not ship static nettle and gcrypt libraries.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Tested-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20190510012458.22706-2-richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

configure

@@ -2784,17 +2784,24 @@ fi
 # GNUTLS probe
 
 if test "$gnutls" != "no"; then
+    pass="no"
     if $pkg_config --exists "gnutls >= 3.1.18"; then
         gnutls_cflags=$($pkg_config --cflags gnutls)
         gnutls_libs=$($pkg_config --libs gnutls)
-        libs_softmmu="$gnutls_libs $libs_softmmu"
-        libs_tools="$gnutls_libs $libs_tools"
-	QEMU_CFLAGS="$QEMU_CFLAGS $gnutls_cflags"
-        gnutls="yes"
-    elif test "$gnutls" = "yes"; then
+        # Packaging for the static libraries is not always correct.
+        # At least ubuntu 18.04 ships only shared libraries.
+        write_c_skeleton
+        if compile_prog "" "$gnutls_libs" ; then
+            libs_softmmu="$gnutls_libs $libs_softmmu"
+            libs_tools="$gnutls_libs $libs_tools"
+            QEMU_CFLAGS="$QEMU_CFLAGS $gnutls_cflags"
+            pass="yes"
+        fi
+    fi
+    if test "$pass" = "no" && test "$gnutls" = "yes"; then
 	feature_not_found "gnutls" "Install gnutls devel >= 3.1.18"
     else
-        gnutls="no"
+        gnutls="$pass"
     fi
 fi
 
@@ -2849,43 +2856,54 @@ has_libgcrypt() {
 
 
 if test "$nettle" != "no"; then
+    pass="no"
     if $pkg_config --exists "nettle >= 2.7.1"; then
         nettle_cflags=$($pkg_config --cflags nettle)
         nettle_libs=$($pkg_config --libs nettle)
         nettle_version=$($pkg_config --modversion nettle)
-        libs_softmmu="$nettle_libs $libs_softmmu"
-        libs_tools="$nettle_libs $libs_tools"
-        QEMU_CFLAGS="$QEMU_CFLAGS $nettle_cflags"
-        nettle="yes"
-
-        if test -z "$gcrypt"; then
-           gcrypt="no"
+        # Link test to make sure the given libraries work (e.g for static).
+        write_c_skeleton
+        if compile_prog "" "$nettle_libs" ; then
+            libs_softmmu="$nettle_libs $libs_softmmu"
+            libs_tools="$nettle_libs $libs_tools"
+            QEMU_CFLAGS="$QEMU_CFLAGS $nettle_cflags"
+            if test -z "$gcrypt"; then
+               gcrypt="no"
+            fi
+            pass="yes"
         fi
+    fi
+    if test "$pass" = "no" && test "$nettle" = "yes"; then
+        feature_not_found "nettle" "Install nettle devel >= 2.7.1"
     else
-        if test "$nettle" = "yes"; then
-            feature_not_found "nettle" "Install nettle devel >= 2.7.1"
-        else
-            nettle="no"
-        fi
+        nettle="$pass"
     fi
 fi
 
 if test "$gcrypt" != "no"; then
+    pass="no"
     if has_libgcrypt; then
         gcrypt_cflags=$(libgcrypt-config --cflags)
         gcrypt_libs=$(libgcrypt-config --libs)
-        # Debian has remove -lgpg-error from libgcrypt-config
+        # Debian has removed -lgpg-error from libgcrypt-config
         # as it "spreads unnecessary dependencies" which in
         # turn breaks static builds...
         if test "$static" = "yes"
         then
             gcrypt_libs="$gcrypt_libs -lgpg-error"
         fi
-        libs_softmmu="$gcrypt_libs $libs_softmmu"
-        libs_tools="$gcrypt_libs $libs_tools"
-        QEMU_CFLAGS="$QEMU_CFLAGS $gcrypt_cflags"
-        gcrypt="yes"
 
+        # Link test to make sure the given libraries work (e.g for static).
+        write_c_skeleton
+        if compile_prog "" "$gcrypt_libs" ; then
+            libs_softmmu="$gcrypt_libs $libs_softmmu"
+            libs_tools="$gcrypt_libs $libs_tools"
+            QEMU_CFLAGS="$QEMU_CFLAGS $gcrypt_cflags"
+            pass="yes"
+        fi
+    fi
+    if test "$pass" = "yes"; then
+        gcrypt="yes"
         cat > $TMPC << EOF
 #include <gcrypt.h>
 int main(void) {
@@ -2898,12 +2916,10 @@ EOF
         if compile_prog "$gcrypt_cflags" "$gcrypt_libs" ; then
             gcrypt_hmac=yes
         fi
+    elif test "$gcrypt" = "yes"; then
+        feature_not_found "gcrypt" "Install gcrypt devel >= 1.5.0"
     else
-        if test "$gcrypt" = "yes"; then
-            feature_not_found "gcrypt" "Install gcrypt devel >= 1.5.0"
-        else
-            gcrypt="no"
-        fi
+        gcrypt="no"
     fi
 fi