I am uploading a NMU to fix this.
The changes are in git and attached as debdiff.
diff -Nru mcpp-2.7.2/debian/changelog mcpp-2.7.2/debian/changelog
--- mcpp-2.7.2/debian/changelog 2024-03-16 13:23:16.000000000 +0000
+++ mcpp-2.7.2/debian/changelog 2024-11-13 21:39:04.000000000 +0000
@@ -1,3 +1,14 @@
+mcpp (2.7.2-5.2) unstable; urgency=medium
+
+ * Non-maintainer upload.
+ * Apply multi-arch hints. + mcpp-doc: Add Multi-Arch: foreign.
+ * Fix build with GCC 14.x (Closes: #1075263).
+
+ [ NIIBE Yutaka ]
+ * Add debian/gbp.conf.
+
+ -- Bastian Germann <b...@debian.org> Wed, 13 Nov 2024 21:39:04 +0000
+
mcpp (2.7.2-5.1) unstable; urgency=medium
* Non-maintainer upload.
diff -Nru mcpp-2.7.2/debian/control mcpp-2.7.2/debian/control
--- mcpp-2.7.2/debian/control 2019-08-15 04:27:10.000000000 +0000
+++ mcpp-2.7.2/debian/control 2024-11-13 21:31:11.000000000 +0000
@@ -49,5 +49,6 @@
Architecture: all
Section: doc
Depends: ${misc:Depends}
+Multi-Arch: foreign
Description: Alternative C/C++ preprocessor (manual)
This package contains the manual.
diff -Nru mcpp-2.7.2/debian/patches/07-fix-16.patch
mcpp-2.7.2/debian/patches/07-fix-16.patch
--- mcpp-2.7.2/debian/patches/07-fix-16.patch 1970-01-01 00:00:00.000000000
+0000
+++ mcpp-2.7.2/debian/patches/07-fix-16.patch 2024-11-13 21:32:59.000000000
+0000
@@ -0,0 +1,22 @@
+Origin:
https://github.com/jbrandwood/mcpp/commit/3b274fe8f31d61996343b17402f30408a6e447cf
+From: John Brandwood <john.brandw...@telzey.com>
+Date: Fri, 15 Mar 2024 15:15:53 -0400
+Subject: Fix build with GCC 14.x by splitting a NULL assignment into 2 lines.
+---
+ src/expand.c | 3 ++-
+ 1 file changed, 2 insertions(+), 1 deletion(-)
+
+diff --git a/src/expand.c b/src/expand.c
+index 08f829a..ea2d2a9 100644
+--- a/src/expand.c
++++ b/src/expand.c
+@@ -710,7 +710,8 @@ static char * replace(
+ } 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; /* Default args */
++ m_inf->loc_args = NULL;
+ for (num = 1, recurs = 0; num < m_num; num++)
+ if (mac_inf[ num].defp == defp)
+ recurs++; /* Recursively nested macro */
diff -Nru mcpp-2.7.2/debian/patches/series mcpp-2.7.2/debian/patches/series
--- mcpp-2.7.2/debian/patches/series 2024-03-16 13:23:16.000000000 +0000
+++ mcpp-2.7.2/debian/patches/series 2024-11-13 21:35:20.000000000 +0000
@@ -4,4 +4,5 @@
04-gniibe-fix-12.patch
05-gniibe-fix-13.patch
06-gniibe-fix-autotools.patch
+07-fix-16.patch
fix-implicit-function-declaration.patch
