configure: add --with-libgcrypt option

libgcrypt is a relatively large dependency that is used only for
the sake of computing MD5 in the CHAP authentication protocol.

Allow distributions to disable it forcibly and rely on the
embedded MD5 implementation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure.ac

@@ -78,8 +78,21 @@ AM_CONDITIONAL([BUILD_EXAMPLES],
 
 AC_CONFIG_HEADERS([config.h])
 
-AC_CHECK_LIB([gcrypt], [gcry_control])
-AM_CONDITIONAL([HAVE_LIBGCRYPT], [test $ac_cv_lib_gcrypt_gcry_control = yes])
+AC_ARG_WITH([libgcrypt],
+	    [AS_HELP_STRING([--with-libgcrypt],
+			    [Use libgcrypt to compute MD5])],
+	    [WITH_LIBGCRYPT=$withval],
+	    [WITH_LIBGCRYPT=auto])
+if test "$WITH_LIBGCRYPT" != no; then
+  AC_CHECK_LIB([gcrypt], [gcry_control])
+  if test "$WITH_LIBGCRYPT" = yes && test "$ac_cv_lib_gcrypt_gcry_control" != yes; then
+    AC_MSG_ERROR([libgcrypt not found])
+  fi
+  WITH_LIBGCRYPT=$ac_cv_lib_gcrypt_gcry_control
+fi
+
+AM_CONDITIONAL([HAVE_LIBGCRYPT],
+	       [expr "$WITH_LIBGCRYPT" : yes > /dev/null 2>&1])
 
 # For MinGW.
 AC_CHECK_LIB([ws2_32], [gethostbyname])