Merge #21421: build: don't try and use -fstack-clash-protection on Windows

7b3434f build: don't try and use -fstack-clash-protection on Windows (fanquake)

Pull request description:

  This has never worked with any of the mingw-w64 compilers we use, and the `-O0` is causing issues for builders applying spectre mitigations (see [IRC logs](http://www.erisian.com.au/bitcoin-core-dev/log-2021-03-12.html#l-15)).

  Recent discussion on https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90458 would also indicate that this should just not be used on Windows.

ACKs for top commit:
  laanwj:
    Concept and code review ACK (but untested) 7b3434f
  hebasto:
    ACK 7b3434f, I've verified that this change does not affect builds for `HOST=x86_64-w64-mingw32` by comparing sizes of the output `*.exe` files.

Tree-SHA512: 72b582321ddff8db3201460fa42a53304e70f141ae078d76a4d4eeb1ca27c8dd567ccb468cc8616179c8df784bd8ca038dcb9a277f9e29f9d98c3cc842916b18

Author: fanquake

configure.ac

@@ -866,11 +866,16 @@ if test x$use_hardening != xno; then
   dnl Use CHECK_LINK_FLAG & --fatal-warnings to ensure we won't use the flag in this case.
   AX_CHECK_LINK_FLAG([-fcf-protection=full],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fcf-protection=full"],, [[$LDFLAG_WERROR]])
 
-  dnl stack-clash-protection does not work properly when building for Windows.
-  dnl We use the test case from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90458
-  dnl to determine if it can be enabled.
-  AX_CHECK_COMPILE_FLAG([-fstack-clash-protection],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-clash-protection"],[],["-O0"],
-    [AC_LANG_SOURCE([[class D {public: unsigned char buf[32768];}; int main() {D d; return 0;}]])])
+  case $host in
+    *mingw*)
+      dnl stack-clash-protection doesn't currently work, and likely should just be skipped for Windows.
+      dnl See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90458 for more details.
+      ;;
+    *)
+      AX_CHECK_COMPILE_FLAG([-fstack-clash-protection],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-clash-protection"])
+      ;;
+  esac
+
 
   dnl When enable_debug is yes, all optimizations are disabled.
   dnl However, FORTIFY_SOURCE requires that there is some level of optimization, otherwise it does nothing and just creates a compiler warning.