libtool: Fix documentation for demo compile mode commands

libtool: Fix documentation for demo compile mode commands

The demo compile mode commands in Chapter 3 fail to compile since
the source files include a header file that is not in the header search
list. The config.h file is in the local folder. Before this fix, the
compile mode commands would return errors like below:

main.c:1:10: fatal error: config.h: No such file or directory
    1 | #include <config.h>
      |          ^~~~~~~~~~
compilation terminated.

The '-I' option has been added to the gcc commands to specify that the
local folder should be searched for header files.

* doc/libtool.texi: Add option to demo compile mode commands