build: pkg anti-pattern: use defaults

Use the defaults in the pkg check where the default error message is
sufficient to identify which package is missing.

Change-Id: If99dcef3cfe918a7d0926cf4fa1eb34f1b619f03
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index fa1700a..f918ba6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -36,8 +36,8 @@
 
     # sdbusplus is non-private because we have no library to link.
     # If we ever have a library, move this to private.
-    AX_PKG_CHECK_MODULES([SDBUSPLUS], [sdbusplus], [], [], [AC_MSG_ERROR(["sdbusplus required and not found."])])
-    AX_PKG_CHECK_MODULES([PHOSPHOR_DBUS_INTERFACES], [], [phosphor-dbus-interfaces], [], [AC_MSG_ERROR(["phosphor-dbus-interfaces required and not found."])])
+    AX_PKG_CHECK_MODULES([SDBUSPLUS], [sdbusplus])
+    AX_PKG_CHECK_MODULES([PHOSPHOR_DBUS_INTERFACES], [phosphor-dbus-interfaces])
 
     # Check for sdbus++
     AC_PATH_PROG([SDBUSPLUSPLUS], [sdbus++])