Add SQUID_MODULE macro (#2124)

Add SQUID_MODULE macro (#2124)

To perform the logic AC_ARG_ENABLE and following checks
for Squid features which have a configurable 'modules'
implementing different behaviours.

Example usage:
```
SQUID_MODULE(foo, [bar baz], [BA variants],[
  Variants of libba*.la to link Squid with.
  Code for these libraries is found in src/foo/
  with one subdirectory per module.
])
```

will define:
 * Makefile.am conditionals;
   ```ENABLE_FOO```, ```ENABLE_FOO_BAR``` and ```ENABLE_FOO_BAZ```

 * Makefile.am variables;
   ```$(FOO_MODULES)``` subdirs that make should recurse into, and
   ```$(FOO_MODULES_LIBS)``` listing the .la files built.

 * C/C++ macros;
   ```USE_FOO```, ```USE_FOO_BAR``` and ```USE_FOO_BAZ```