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: Ib4cb64967932bcf4635fac69af0c382aff9931e6
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index 979802c..fc153a4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -27,9 +27,8 @@
         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])
 
     # Check for baseline language coverage in the compiler for the C++14 standard
     AX_CXX_COMPILE_STDCXX_17([noext])