GCC 14 treats the following situations, which were only warnings
before, as errors:
  -Werror=implicit-int
  -Werror=implicit-function-declaration
  -Werror=declaration-missing-parameter-type
  -Werror=return-mismatch
  -Werror=int-conversion
  -Werror=incompatible-pointer-types

More details about all of these cases together with the most common
ways of addressing them and some associated pitfalls is available at
https://gcc.gnu.org/gcc-14/porting_to.html

For the record, clang has taken a similar step since clang 16, see
for example https://www.redhat.com/en/blog/new-warnings-and-errors-clang-16

The preferred method of dealing with these issues is of course fixing
the code.  Other distributions have shared patches from their porting
efforts on relevant upstream mailing lists and bug trackers, Gentoo
and RedHat are perhaps most active in this regard, so you can have a
look at what they did for their packages at:
  - https://bugs.gentoo.org/showdependencytree.cgi?id=870412&hide_resolved=0
  - https://gitlab.com/fweimer-rh/fedora-modernc/

In case there is for some reason not possible to fix the code, or at
least to fix it in time for adoption of GCC 14, it is possible to turn
these back to warnings by adding to the compiler options one of the
following:
  -std=gnu89,
  -std=c89, or
  -fpermissive.

(Especially if you use -fpermissive as a temporary workaround with
intention to fix the underlying issues later, I'd suggest leaving the
particular bug opened until it is done so.)