Assorted fixes for GCC 15.

Upstream bug: https://sourceforge.net/p/mcpp/bugs/18/

--- mcpp-2.7.2/configure.ac	2008-11-26 09:08:29.000000000 +0000
+++ mcpp-2.7.2/configure.ac	2025-07-03 18:33:29.088160597 +0100
@@ -152,7 +152,7 @@
 
 dnl Checks for header files.
 
-AC_CHECK_HEADERS( [unistd.h, stdint.h, inttypes.h])
+AC_CHECK_HEADERS( [unistd.h stdint.h inttypes.h])
 
 dnl Checks for typedefs, and compiler characteristics.
 
@@ -207,8 +207,8 @@
         AC_RUN_IFELSE( [[
 #include    <stdio.h>
 #include    <assert.h>
-    int     strcmp();
-    void    exit();
+#include    <string.h>
+#include    <stdlib.h>
 $3
 int main( void)
 {   char    buf@<:@ 20@:>@;
--- mcpp-2.7.2/src/configed.H	2008-11-20 07:51:38.000000000 +0000
+++ mcpp-2.7.2/src/configed.H	2025-07-03 18:39:33.490676384 +0100
@@ -295,20 +295,7 @@
  * ULONGMAX     should be defined to the ULONG_MAX in <limits.h>.
  */
 
-/* _POSIX_* only to get PATH_MAX    */
-#define _POSIX_             1
-#define _POSIX_SOURCE       1
-#ifndef _POSIX_C_SOURCE
-#define _POSIX_C_SOURCE     1
-#define _POSIX_C_SOURCE_defined     1
-#endif
 #include    "limits.h"
-#undef  _POSIX_
-#undef  _POSIX_SOURCE
-#ifdef  _POSIX_C_SOURCE_defined
-#undef  _POSIX_C_SOURCE
-#undef  _POSIX_C_SOURCE_defined
-#endif
 #define CHARBIT             CHAR_BIT
 #define UCHARMAX            UCHAR_MAX
 #define USHRTMAX            USHRT_MAX
--- mcpp-2.7.2/src/expand.c	2008-03-18 08:50:41.000000000 +0000
+++ mcpp-2.7.2/src/expand.c	2025-07-03 18:34:00.429141160 +0100
@@ -710,7 +710,8 @@
         } else {
             m_inf->locs.start_col = m_inf->locs.start_line = 0L;
         }
-        m_inf->args = m_inf->loc_args = NULL;       /* Default args */
+        m_inf->args = NULL;
+        m_inf->loc_args = NULL;       /* Default args */
         for (num = 1, recurs = 0; num < m_num; num++)
             if (mac_inf[ num].defp == defp)
                 recurs++;           /* Recursively nested macro     */
--- mcpp-2.7.2/src/system.c	2008-11-26 09:53:51.000000000 +0000
+++ mcpp-2.7.2/src/system.c	2025-07-03 18:40:12.426621354 +0100
@@ -3431,7 +3431,7 @@
     if (! fullname)                 /* Non-existent or directory    */
         return  FALSE;
     if (standard && included( fullname))        /* Once included    */
-        goto  true;
+        goto  ltrue;
         
     if ((max_open != 0 && max_open <= include_nest)
                             /* Exceed the known limit of open files */
@@ -3458,12 +3458,12 @@
         if ((fp = fopen( fullname, "r")) == NULL) {
             file->fp = fopen( cur_fullname, "r");
             fseek( file->fp, file->pos, SEEK_SET);
-            goto  false;
+            goto  lfalse;
         }
         if (max_open == 0)      /* Remember the limit of the system */
             max_open = include_nest;
     } else if (fp == NULL)                  /* No read permission   */ 
-        goto  false;
+        goto  lfalse;
     /* Truncate buffer of the includer to save memory   */
     len = (int) (file->bptr - file->buffer);
     if (len) {
@@ -3510,9 +3510,9 @@
     if (mkdep && ((mkdep & MD_SYSHEADER) || ! infile->sys_header))
         put_depend( fullname);          /* Output dependency line   */
 
-true:
+ltrue:
     return  TRUE;
-false:
+lfalse:
     free( fullname);
     return  FALSE;
 }