build-sys: Add check-lib target

The referenced commits created the library infrastructure and test
program to validate that the structures and macros line up with
each other.

The library needs to be (re)built with -DITEMTABLE_DEBUG and then
the test program ran.  We clean before and after so we are not
testing a non-debug library or having a debug library hanging around
to cause future problems.

Due to test_Itemtables depending on the library, we don't need to
explicitly build the library.

To validate the library structure/header corrospondence run:
make check-lib

References:
 commit e616409aa4
 commit 92d0297e1e
 https://www.freelists.org/post/procps/keep-on-patchin,19

Makefile.am

@@ -371,9 +371,13 @@ endif
 
 BUILT_SOURCES = $(top_srcdir)/.version
 
+check-lib: clean
+	$(MAKE) CFLAGS=-DITEMTABLE_DEBUG proc/test_Itemtables
+	$(top_builddir)/proc/test_Itemtables
+	$(MAKE) clean
+
 # Test programs not used by dejagnu but run directly
 TESTS = \
-	proc/test_Itemtables \
 	proc/test_pids \
 	proc/test_uptime \
 	proc/test_sysinfo \