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: Ibabd53962baedda86b53bb0bdadf936d0036af63
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index 3056a7b..2fafab1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -28,8 +28,8 @@
         AC_MSG_ERROR([Cannot find sdbus++]))
 
     # Checks for libraries.
-    AX_PKG_CHECK_MODULES([SYSTEMD], [], [libsystemd >= 221], [], [AC_MSG_ERROR(["systemd required and not found."])])
-    AX_PKG_CHECK_MODULES([SDBUSPLUS], [], [sdbusplus], [], [AC_MSG_ERROR(["sdbusplus required and not found."])])
+    AX_PKG_CHECK_MODULES([SYSTEMD], [], [libsystemd >= 221])
+    AX_PKG_CHECK_MODULES([SDBUSPLUS], [], [sdbusplus])
 
     # Checks for typedefs, structures, and compiler characteristics.
     AX_CXX_COMPILE_STDCXX_17([noext])