libpng, zsh: two more GCC 14+ strictness regressions

libpng
  contrib/libtests/pngvalid.c calls feenableexcept(), a glibc extension
  relibc does not provide, and GCC 14+ makes the implicit declaration an
  error. It is a test program, so build it the way AGENTS.md
  § CONVENTIONS already prescribes for cross builds -- '--disable-tests'.
  The library is unaffected; only check_PROGRAMS is skipped, and those
  binaries cannot run on the build host regardless.

zsh
  zsh 5.9 probes for the termcap symbol arrays with
      char **test = boolcodes; puts(*test);
  and ncurses declares boolcodes as 'const char * const *'. GCC 14
  promoted -Wincompatible-pointer-types to an error, so the probe stopped
  compiling, configure concluded boolcodes=no, HAVE_BOOLCODES was left
  undefined, and Src/Modules/termcap.c fell back to defining its own
  array -- colliding with the header:
      termcap.c:45: error: conflicting types for 'boolcodes'
  The probe uses a bare shell variable, not an ac_cv_ cache entry, so
  there is nothing to override; restoring the pre-GCC-14 severity is what
  lets it reach the right answer. Scoped to this recipe so the rest of the
  tree keeps the stricter default.

Both cook clean.

recipes/libs/libpng/recipe.toml

@@ -17,5 +17,17 @@ template = "custom"
 dependencies = ["zlib"]
 script = """
 DYNAMIC_STATIC_INIT
+# Do not build the test harness when cross-compiling, per AGENTS.md
+# § CONVENTIONS ("Cross-compilation tests: Disable test suites when
+# cross-compiling"). contrib/libtests/pngvalid.c calls feenableexcept(),
+# a glibc extension relibc does not provide, so with GCC 14+ turning
+# implicit declarations into errors the test program fails the build:
+#   pngvalid.c:11716: error: implicit declaration of function
+#   'feenableexcept'
+# The library itself is unaffected -- only check_PROGRAMS is skipped, and
+# those binaries cannot run on the build host anyway.
+COOKBOOK_CONFIGURE_FLAGS+=(
+    --disable-tests
+)
 cookbook_configure
 """

recipes/shells/zsh/recipe.toml

@@ -21,6 +21,18 @@ dependencies = [
 script = """
 DYNAMIC_INIT
 
+# zsh 5.9 predates GCC 14, which promoted -Wincompatible-pointer-types from a
+# warning to an error. Its configure probe for the termcap symbol arrays is
+#     char **test = boolcodes; puts(*test);
+# and ncurses declares boolcodes as `const char * const *`, so under GCC 16 the
+# probe no longer compiles. configure then concludes boolcodes=no, leaves
+# HAVE_BOOLCODES undefined, and Src/Modules/termcap.c falls back to defining
+# its own array -- which collides with the header:
+#     termcap.c:45: error: conflicting types for 'boolcodes'; have 'char *[]'
+# Restore the pre-GCC-14 severity so the probes reach the right answers. Scoped
+# to this recipe: the rest of the tree keeps the stricter default.
+export CFLAGS+=" -Wno-error=incompatible-pointer-types"
+
 COOKBOOK_CONFIGURE_FLAGS+=(
     --srcdir="${COOKBOOK_SOURCE}"
     --disable-gdbm