Fix compilation on C2X/C23 compilers

[laurensmiers]

MCPP fails to compile due to stricter checks in newer compiles.

First error is regarding a mismatch of argument expectations:

mcpp_main.c: In function ‘mcpp_lib_main’:
mcpp_main.c:305:5: error: too many arguments to function ‘mb_init’; expected 0, have 1
  305 |     mb_init(TRUE);      /* Should be initialized prior to get options   */
      |     ^~~~~~~
In file included from mcpp_main.c:39:
internal.H:419:17: note: declared here
  419 | extern void     mb_init();
      |                 ^~~~~~~
make: *** [<builtin>: mcpp_main.o] Error 1

Second error is regarding the use of reserved keywords for goto-labels:

system.c: In function ‘open_file’:
system.c:1741:15: error: expected identifier or ‘*’ before ‘true’
 1741 |         goto  true;
      |               ^~~~
system.c:1768:19: error: expected identifier or ‘*’ before ‘false’
 1768 |             goto  false;
      |                   ^~~~~
system.c:1773:15: error: expected identifier or ‘*’ before ‘false’
 1773 |         goto  false;
      |               ^~~~~
system.c:1805:5: error: expected ‘;’ before ‘:’ token
 1805 | true:
      |     ^
      |     ;
system.c:1807:6: error: expected ‘;’ before ‘:’ token
 1807 | false:
      |      ^
      |      ;
make: *** [<builtin>: system.o] Error 1

When both of these are tackled, mcpp compilation succeeds using GCC 15.22.1:

$ cc --version
cc (GCC) 15.2.1 20250813
$ make
$ ls lib64/libmcpp.a
lib64/libmcpp.a

This was also flagged in the original mcpp, but no fix accepted yet:
https://sourceforge.net/p/mcpp/bugs/18/

This fix is necessary if we want to install zeroc-ice pypi package since on Linux, the backend is build and these errors pop-up.

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