configure: Don't stop when cppunit is not found

configure.ac

@@ -202,7 +202,11 @@ fi
 # Checks for libraries.
 
 # Check availability of cppunit
-PKG_CHECK_MODULES([CPPUNIT], [cppunit >= 1.10.2], [], [])
+PKG_CHECK_MODULES([CPPUNIT], [cppunit >= 1.10.2],
+                  [have_cppunit=yes], [have_cppunit=no])
+if test "x$have_cppunit" != "xyes"; then
+  AC_MSG_WARN([$CPPUNIT_PKG_ERRORS])
+fi
 
 # Check availability of libz
 if test "x$with_libz" = "xyes"; then