Unconditionally add -Wextra GCC/Clang option and remove GCC_WEXTRA macro

ChangeLog

@@ -1,3 +1,12 @@
+2024-08-26 Roy Hills <royhills@hotmail.com>
+
+	* configure.ac: Unconditionally add -Wextra to GCC and Clang compiler
+	  options instead of using the GCC_WEXTRA macro. -Wextra was added in
+	  GCC 3.4.0, which is 20 years old now, and has always been supported
+	  by Clang. There are no supported systems with earlier GCC versions.
+
+	* m4/gcc-wextra.m4: Removed.
+
 2024-08-14 Roy Hills <royhills@hotmail.com>
 
 	* get-iab: removed. This script is no longer needed and was depreciated

configure.ac

@@ -29,17 +29,19 @@ AC_ARG_ENABLE(gcov,
       AC_MSG_RESULT(no)
    ] )
 
-# Checks for programs.
+# Check for utility programs that we need.
 AC_PROG_SED
 AC_PROG_CC
+# Ensure the C compiler supports the C99 standard.
 if test "x$ac_cv_prog_cc_c99" = "xno"; then
    AC_MSG_ERROR([C compiler does not support C99 standard])
 fi
+# Add additional options if the C compiler identifies as GCC.
+# This applies to Clang/LLVM in addition to GCC.
 if test -n "$GCC"; then
    AC_DEFINE([ATTRIBUTE_UNUSED], [__attribute__ ((__unused__))],
              [Define to the compiler's unused pragma])
-   CFLAGS="$CFLAGS -Wall -Wshadow -Wwrite-strings"
-   GCC_WEXTRA
+   CFLAGS="$CFLAGS -Wall -Wextra -Wshadow -Wwrite-strings"
    GCC_STACK_PROTECT_CC
    GCC_FORTIFY_SOURCE
    GCC_FORMAT_SECURITY