Add SQUID_CHECK_LIB_WORKS macro (#1333)

Add SQUID_CHECK_LIB_WORKS macro (#1333)

... to run validation for a library after SQUID_AUTO_LIB detection.

A typical use of this macro could be:
```
  SQUID_AUTO_LIB(foo,[Foo],[LIBFOO])
  SQUID_CHECK_LIB_WORKS(foo,[
    PKG_CHECK_MODULES([LIBFOO],[foo >= 1.0],[],[
      LIBFOO_LIBS=""
    ])
    AC_CHECK_HEADERS([foo.h],[],[LIBFOO_LIBS=""])
  ])
```

Update of configure.ac to use this macro uncovered and
fixed a bug which may have broken libnettle detection on
some systems.