Merge bitcoin/bitcoin#21788: build: Silence [-Wunused-command-line-argument] warnings

e9f948c build: Convert warnings into errors when testing for -fstack-clash-protection (Hennadii Stepanov)

Pull request description:

  Apple clang version 12.0.5 (clang-1205.0.22.9) that is a part of Xcode 12.5, and is based on LLVM clang 11.1.0, fires spammy warnings:

  ```
  clang: warning: argument unused during compilation: '-fstack-clash-protection' [-Wunused-command-line-argument]
  ```

  From the https://github.com/apple/llvm-project:
  ```
  $ git log --oneline | grep 'stack-clash-protection'
  00065d5cbd02 Revert "-fstack-clash-protection: Return an actual error when used on unsupported OS"
  4d59c8fdb955 -fstack-clash-protection: Return an actual error when used on unsupported OS
  df3bfaa39071 [Driver] Change -fnostack-clash-protection to  -fno-stack-clash-protection
  68e07da3e5d5 [clang][PowerPC] Enable -fstack-clash-protection option for ppc64
  515bfc66eace [SystemZ] Implement -fstack-clash-protection
  e67cbac81211 Support -fstack-clash-protection for x86
  454621160066 Revert "Support -fstack-clash-protection for x86"
  0fd51a4554f5 Support -fstack-clash-protection for x86
  658495e6ecd4 Revert "Support -fstack-clash-protection for x86"
  e229017732bc Support -fstack-clash-protection for x86
  b03c3d8c6209 Revert "Support -fstack-clash-protection for x86"
  4a1a0690ad68 Support -fstack-clash-protection for x86
  f6d98429fcdb Revert "Support -fstack-clash-protection for x86"
  39f50da2a357 Support -fstack-clash-protection for x86
  ```

  I suppose, that Apple clang-1205.0.22.9 ends with on of the "Revert..." commits.

  This PR prevents using of the `-fstack-clash-protection` flag if it causes warnings.

  ---

  System: macOS Big Sur 11.3 (20E232).

ACKs for top commit:
  jarolrod:
    re-ACK e9f948c
  Sjors:
    tACK e9f948c on macOS 11.3.1

Tree-SHA512: 30186da67f9b0f34418014860c766c2e7f622405520f1cbbc1095d4aa4038b0a86014d76076f318a4b1b09170a96d8167c21d7f53a760e26017f486e1a7d39d4

Author: fanquake

configure.ac

@@ -888,7 +888,7 @@ if test x$use_hardening != xno; then
       dnl See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90458 for more details.
       ;;
     *)
-      AX_CHECK_COMPILE_FLAG([-fstack-clash-protection],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-clash-protection"])
+      AX_CHECK_COMPILE_FLAG([-fstack-clash-protection], [HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-clash-protection"], [], [$CXXFLAG_WERROR])
       ;;
   esac