cmake: Fix TryAppendCXXFlags module

[hebasto]

This PR fixes a couple of bugs in the TryAppendCXXFlags module that I noticed during testing on the staging branch:

1. Fixes the check of multiple flags. As documented, a space must be used as a separator:

   bitcoin/cmake/module/TryAppendCXXFlags.cmake

   Lines 12 to 14 in 56450e5

   	Usage examples:
   	try_append_cxx_flags("-Wformat -Wformat-security" VAR warn_cxx_flags)

Otherwise, only the first flag is actually being checked.

Here are excerpts from build/CMakeFiles/CMakeConfigureLog.yaml:

• on the staging branch @ 9256718:

    kind: "try_compile-v1"
    backtrace:
      - "/usr/share/cmake-3.27/Modules/Internal/CheckSourceCompiles.cmake:101 (try_compile)"
      - "/usr/share/cmake-3.27/Modules/CheckCXXSourceCompiles.cmake:52 (cmake_check_source_compiles)"
      - "cmake/module/TryAppendCXXFlags.cmake:70 (check_cxx_source_compiles)"
      - "CMakeLists.txt:416 (try_append_cxx_flags)"
    checks:
      - "Performing Test CXX_SUPPORTS__WFORMAT__WFORMAT_SECURITY"
    directories:
      source: "/home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-8Q9qSw"
      binary: "/home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-8Q9qSw"
    cmakeVariables:
      CMAKE_CXX_FLAGS: ""
      CMAKE_CXX_FLAGS_DEBUG: "-g"
      CMAKE_CXX_LINK_NO_PIE_SUPPORTED: "1"
      CMAKE_EXE_LINKER_FLAGS: ""
      CMAKE_MODULE_PATH: "/home/hebasto/git/bitcoin/cmake/module"
      CMAKE_POSITION_INDEPENDENT_CODE: "ON"
    buildResult:
      variable: "CXX_SUPPORTS__WFORMAT__WFORMAT_SECURITY"
      cached: true
      stdout: |
        Change Dir: '/home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-8Q9qSw'
        
        Run Build Command(s): /usr/bin/cmake -E env VERBOSE=1 /usr/bin/gmake -f Makefile cmTC_52a2c/fast
        /usr/bin/gmake  -f CMakeFiles/cmTC_52a2c.dir/build.make CMakeFiles/cmTC_52a2c.dir/build
        gmake[1]: Entering directory '/home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-8Q9qSw'
        Building CXX object CMakeFiles/cmTC_52a2c.dir/src.cxx.o
        /usr/bin/c++ -DCXX_SUPPORTS__WFORMAT__WFORMAT_SECURITY  -Wformat -std=c++20 -fPIC -o CMakeFiles/cmTC_52a2c.dir/src.cxx.o -c /home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-8Q9qSw/src.cxx
        Linking CXX static library libcmTC_52a2c.a
        /usr/bin/cmake -P CMakeFiles/cmTC_52a2c.dir/cmake_clean_target.cmake
        /usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_52a2c.dir/link.txt --verbose=1
        /usr/bin/ar qc libcmTC_52a2c.a CMakeFiles/cmTC_52a2c.dir/src.cxx.o
        /usr/bin/ranlib libcmTC_52a2c.a
        gmake[1]: Leaving directory '/home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-8Q9qSw'
        
      exitCode: 0

• with this PR:

    kind: "try_compile-v1"
    backtrace:
      - "/usr/share/cmake-3.27/Modules/Internal/CheckSourceCompiles.cmake:101 (try_compile)"
      - "/usr/share/cmake-3.27/Modules/CheckCXXSourceCompiles.cmake:52 (cmake_check_source_compiles)"
      - "cmake/module/TryAppendCXXFlags.cmake:70 (check_cxx_source_compiles)"
      - "CMakeLists.txt:416 (try_append_cxx_flags)"
    checks:
      - "Performing Test CXX_SUPPORTS__WFORMAT__WFORMAT_SECURITY"
    directories:
      source: "/home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-SK4Pzn"
      binary: "/home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-SK4Pzn"
    cmakeVariables:
      CMAKE_CXX_FLAGS: ""
      CMAKE_CXX_FLAGS_DEBUG: "-g"
      CMAKE_CXX_LINK_NO_PIE_SUPPORTED: "1"
      CMAKE_EXE_LINKER_FLAGS: ""
      CMAKE_MODULE_PATH: "/home/hebasto/git/bitcoin/cmake/module"
      CMAKE_POSITION_INDEPENDENT_CODE: "ON"
    buildResult:
      variable: "CXX_SUPPORTS__WFORMAT__WFORMAT_SECURITY"
      cached: true
      stdout: |
        Change Dir: '/home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-SK4Pzn'
        
        Run Build Command(s): /usr/bin/cmake -E env VERBOSE=1 /usr/bin/gmake -f Makefile cmTC_ef70a/fast
        /usr/bin/gmake  -f CMakeFiles/cmTC_ef70a.dir/build.make CMakeFiles/cmTC_ef70a.dir/build
        gmake[1]: Entering directory '/home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-SK4Pzn'
        Building CXX object CMakeFiles/cmTC_ef70a.dir/src.cxx.o
        /usr/bin/c++ -DCXX_SUPPORTS__WFORMAT__WFORMAT_SECURITY  -Wformat -Wformat-security -Werror -std=c++20 -fPIC -o CMakeFiles/cmTC_ef70a.dir/src.cxx.o -c /home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-SK4Pzn/src.cxx
        Linking CXX static library libcmTC_ef70a.a
        /usr/bin/cmake -P CMakeFiles/cmTC_ef70a.dir/cmake_clean_target.cmake
        /usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_ef70a.dir/link.txt --verbose=1
        /usr/bin/ar qc libcmTC_ef70a.a CMakeFiles/cmTC_ef70a.dir/src.cxx.o
        /usr/bin/ranlib libcmTC_ef70a.a
        gmake[1]: Leaving directory '/home/hebasto/git/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-SK4Pzn'
        
      exitCode: 0

2. Avoid skipping checks for linking due to caching of the result variable. The effect can be observed in the configuration output.

[m3dwards]

ACK 634b3c7

I can replicate the problem without the PR and see that it's fixed with the PR.

cat build/CMakeFiles/CMakeConfigureLog.yaml | grep format-security
        /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -DCXX_SUPPORTS__WFORMAT__WFORMAT_SECURITY  -Wformat -Wformat-security -Werror -std=c++20 -arch arm64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.5.sdk -fPIC -MD -MT CMakeFiles/cmTC_9f576.dir/src.cxx.o -MF CMakeFiles/cmTC_9f576.dir/src.cxx.o.d -o CMakeFiles/cmTC_9f576.dir/src.cxx.o -c /Users/maxedwards/source/bitcoin/build/CMakeFiles/CMakeScratch/TryCompile-IECvyk/src.cxx