build: use _FORTIFY_SOURCE=3

glibc 2.33 introduced a new fortification level, _FORTIFY_SOURCE=3.
Which improves the coverage of cases where _FORTIFY_SOURCE can use _chk
functions. For example, using GCC 13 and glibc 2.36 (Fedora Rawhide),
compiling master:
```bash
nm -C src/bitcoind | grep _chk
                 U __fprintf_chk@GLIBC_2.17
                 U __memcpy_chk@GLIBC_2.17
                 U __snprintf_chk@GLIBC_2.17
                 U __sprintf_chk@GLIBC_2.17
                 U __stack_chk_fail@GLIBC_2.17
                 U __stack_chk_guard@GLIBC_2.17
                 U __vsnprintf_chk@GLIBC_2.17

objdump -d src/bitcoind | grep "_chk@plt" | wc -l
33
```

vs this branch:
```bash
nm -C src/bitcoind | grep _chk
                 U __fprintf_chk@GLIBC_2.17
                 U __memcpy_chk@GLIBC_2.17
                 U __memset_chk@GLIBC_2.17
                 U __snprintf_chk@GLIBC_2.17
                 U __sprintf_chk@GLIBC_2.17
                 U __stack_chk_fail@GLIBC_2.17
                 U __stack_chk_guard@GLIBC_2.17
                 U __vsnprintf_chk@GLIBC_2.17

objdump -d src/bitcoind | grep "_chk@plt" | wc -l
61
```

Usage of level 3 requires LLVM/Clang 9+, or GCC 12+. Older
compilers/glibc will still use _FORTIFY_SOURCE=2. For example, in the
glibc we currently use for Linux release builds (2.24), FORTIFY_LEVEL is
determined using the following:
```c
```
so any value > 1 will turn on _FORTIFY_SOURCE=2.

https://sourceware.org/pipermail/libc-alpha/2021-February/122207.html
https://developers.redhat.com/blog/2021/04/16/broadening-compiler-checks-for-buffer-overflows-in-_fortify_source

Author: fanquake

configure.ac

@@ -963,11 +963,11 @@ if test "$use_hardening" != "no"; then
   dnl However, FORTIFY_SOURCE requires that there is some level of optimization, otherwise it does nothing and just creates a compiler warning.
   dnl Since FORTIFY_SOURCE is a no-op without optimizations, do not enable it when enable_debug is yes.
   if test "$enable_debug" != "yes"; then
-    AX_CHECK_PREPROC_FLAG([-D_FORTIFY_SOURCE=2],[
+    AX_CHECK_PREPROC_FLAG([-D_FORTIFY_SOURCE=3],[
       AX_CHECK_PREPROC_FLAG([-U_FORTIFY_SOURCE],[
         HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -U_FORTIFY_SOURCE"
       ])
-      HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -D_FORTIFY_SOURCE=2"
+      HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -D_FORTIFY_SOURCE=3"
     ])
   fi