use cmake conditional for gcc < 9 instead

Pretty sure this works, preliminary testing.
Feel free to revert if not

libksysguard-5.22.2.1_fix-processcore-on-gcc8.patch

@@ -15,7 +15,7 @@ index c28d364..d3da632 100644
      KF5::CoreAddons
      KF5::Service
      ZLIB::ZLIB
-+    stdc++fs
++    $<$<AND:$<CXX_COMPILER_ID:GNU>,$<VERSION_LESS:$<CXX_COMPILER_VERSION>,9.0>>:stdc++fs>
  )
  
  if( ${CMAKE_SYSTEM_NAME} MATCHES "NetBSD" )

libksysguard.spec

@@ -16,7 +16,6 @@ URL:     https://invent.kde.org/plasma/%{name}
 Source0: http://download.kde.org/%{stable}/plasma/%{version}/%{name}-%{version}.tar.xz
 
 # GCC 8 and older need stdc++fs link library set
-# GCC 9+ have it set by default.  It does not harm to set it again.
 Patch1:            libksysguard-5.22.2.1_fix-processcore-on-gcc8.patch
 
 BuildRequires:  extra-cmake-modules