build: add -std=gnu17 for the GCC 16 / C23 build

GCC 16 defaults to C23, where "()" in a function declaration means
"(void)" rather than "unspecified arguments".  That breaks the many
legacy K&R-style "()" declarations still present throughout the tree,
independently of the K&R-to-ANSI conversion that follows in subsequent
per-directory commits.

Add -std=gnu17 (single dash; clang silently ignores the "--std=" long
form some CI scripts pass) in the gcc SHLIB_CFLAGS block and the
emscripten target to restore C17 semantics, and drop the now-unnecessary
-Wno-implicit-int suppression from the emscripten target.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/configure

@@ -8747,7 +8747,11 @@ case $target in
   *-emscripten*)
     $as_echo "#define linux 1" >>confdefs.h
 
-    CFLAGS="${CFLAGS} -fPIC -Werror=implicit-function-declaration -Wno-int-conversion -Wno-implicit-int"
+    # emcc (clang) defaults to C23, where "()" means "(void)".  Force
+    # -std=gnu17 to keep the legacy "()" declarations compiling, matching
+    # the native gcc build.  (-std=gnu17 must be a single dash; clang
+    # silently ignores the "--std=" long form used in some CI scripts.)
+    CFLAGS="${CFLAGS} -std=gnu17 -fPIC -Werror=implicit-function-declaration -Wno-int-conversion"
     ;;
   *solaris*)
     $as_echo "#define SYSV 1" >>confdefs.h
@@ -9189,7 +9193,10 @@ fi
       *cygwin*)
         ;;
       *)
-        SHLIB_CFLAGS="-Wimplicit-int -fPIC"
+        # GCC 16 defaults to C23, where "()" means "(void)".  Magic relies on
+        # the older C17 semantics where "()" means "unspecified arguments".
+        # Force -std=gnu17 to keep the legacy declarations compiling.
+        SHLIB_CFLAGS="-std=gnu17 -Wimplicit-int -fPIC"
         ;;
     esac
   fi

scripts/configure.in

@@ -1471,7 +1471,11 @@ case $target in
     ;; 
   *-emscripten*)
     AC_DEFINE(linux)
-    CFLAGS="${CFLAGS} -fPIC -Werror=implicit-function-declaration -Wno-int-conversion -Wno-implicit-int"
+    dnl emcc (clang) defaults to C23, where "()" means "(void)".  Force
+    dnl -std=gnu17 to keep the legacy "()" declarations compiling, matching
+    dnl the native gcc build.  (-std=gnu17 must be a single dash; clang
+    dnl silently ignores the "--std=" long form used in some CI scripts.)
+    CFLAGS="${CFLAGS} -std=gnu17 -fPIC -Werror=implicit-function-declaration -Wno-int-conversion"
     ;;
   *solaris*)
     AC_DEFINE(SYSV)
@@ -1854,7 +1858,10 @@ if test $usingTcl ; then
       *cygwin*)
         ;;
       *)
-        SHLIB_CFLAGS="-Wimplicit-int -fPIC"
+        dnl GCC 16 defaults to C23, where "()" means "(void)".  Magic relies on
+        dnl the older C17 semantics where "()" means "unspecified arguments".
+        dnl Force -std=gnu17 to keep the legacy declarations compiling.
+        SHLIB_CFLAGS="-std=gnu17 -Wimplicit-int -fPIC"
         ;;
     esac
   fi