CMAKE_EXPORT_COMPILE_COMMANDS: Implicit include directories and compile_commands.json

When using a g++ compiler for build, the clang-tidy program does not find the c++ header.

Claus-iMac:netkit-tftp clausklein$ g++-9 -x c++ -Wp,-v /dev/null
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/Cellar/gcc/9.3.0_1/lib/gcc/9/gcc/x86_64-apple-darwin19/9.3.0/../../../../../../include/c++/9.3.0
 /usr/local/Cellar/gcc/9.3.0_1/lib/gcc/9/gcc/x86_64-apple-darwin19/9.3.0/../../../../../../include/c++/9.3.0/x86_64-apple-darwin19
 /usr/local/Cellar/gcc/9.3.0_1/lib/gcc/9/gcc/x86_64-apple-darwin19/9.3.0/../../../../../../include/c++/9.3.0/backward
 /usr/local/Cellar/gcc/9.3.0_1/lib/gcc/9/gcc/x86_64-apple-darwin19/9.3.0/include
 /usr/local/Cellar/gcc/9.3.0_1/lib/gcc/9/gcc/x86_64-apple-darwin19/9.3.0/include-fixed
 /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/include
 /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks
End of search list.

see too https://discourse.cmake.org/t/system-include-path/1457?u=clausklein