During development it is often handy to change the config.h options from
their defaults, for example to enable debugging features.

To prevent accidental commits of debugging config.h changes, mdc
suggested having a config-local.h that is excluded from source control.
This file acts as a temporary config.h and can override any of the
defaults.

This commit is an attempt to implement the config-local.h feature.

The config.h file now has the following as its last line:
/* @TRYSOURCE config-local.h */

The @TRYSOURCE directive causes config-local.h to be included at that
point in the file.  If config-local.h does not exist, no error will be
printed and parsing will continue as normal.  Therefore, mkconfig.pl is
"trying" to "source" config-local.h.