clang - Cross compile for linux on macOS - c

Question

I am trying to cross compile for linux on macOS. It appears that clang is not respecting the sysroot I set.
Program
#include <stdio.h>
int main() {
printf("Hello World!\n");
return 0;
}
Commands
CC=/Users/user/Development/com/prog/site_scons/builders/darwin/clang+llvm-9.0.0-x86_64-darwin-apple/bin/clang
${CC} -v -c main.c -target x86_64-pc-linux-elf -o main.o\
-isysroot /Users/zrgorak/Development/com/prog/site_scons/builders/linux/sysroot
${CC} -v -o main -target x86_64-pc-linux-elf main.o \
-isysroot /Users/zrgorak/Development/com/prog/site_scons/builders/linux/sysroot
Output
% make
/Users/user/Development/com/prog/site_scons/builders/darwin/clang+llvm-9.0.0-x86_64-darwin-apple/bin/clang -v -c -target x86_64-pc-linux-elf main.c -o main.o \
-isysroot /Users/user/Development/com/prog/site_scons/builders/linux/sysroot
clang version 9.0.0 (git://github.com/llvm/llvm-project.git 0399d5a9682b3cef71c653373e38890c63c4c365)
Target: x86_64-pc-linux-elf
Thread model: posix
InstalledDir: /Users/user/Development/com/prog/site_scons/builders/darwin/clang+llvm-9.0.0-x86_64-darwin-apple/bin
"/Users/user/Development/com/prog/site_scons/builders/darwin/clang+llvm-9.0.0-x86_64-darwin-apple/bin/clang-9" -cc1 -triple x86_64-pc-linux-elf -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name main.c -mrelocation-model static -mthread-model posix -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -dwarf-column-info -debugger-tuning=gdb -target-linker-version 512.4 -v -coverage-notes-file /Users/user/Development/testlinker/main.gcno -resource-dir /Users/user/Development/com/prog/site_scons/builders/darwin/clang+llvm-9.0.0-x86_64-darwin-apple/lib/clang/9.0.0 -isysroot /Users/user/Development/com/prog/site_scons/builders/linux/sysroot -internal-isystem /usr/local/include -internal-isystem /Users/user/Development/com/prog/site_scons/builders/darwin/clang+llvm-9.0.0-x86_64-darwin-apple/lib/clang/9.0.0/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdebug-compilation-dir /Users/user/Development/testlinker -ferror-limit 19 -fmessage-length 128 -fobjc-runtime=gcc -fdiagnostics-show-option -fcolor-diagnostics -faddrsig -o main.o -x c main.c
clang -cc1 version 9.0.0 based upon LLVM 9.0.0 default target x86_64-apple-darwin19.0.0
ignoring nonexistent directory "/include"
ignoring nonexistent directory "/usr/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/local/include
/Users/user/Development/com/prog/site_scons/builders/darwin/clang+llvm-9.0.0-x86_64-darwin-apple/lib/clang/9.0.0/include
End of search list.
main.c:1:10: fatal error: 'stdio.h' file not found
#include <stdio.h>
^~~~~~~~~
1 error generated.
make: *** [target] Error 1
My sysroot is /usr and /lib copied from Ubuntu 19.x
% ll /Users/user/Development/com/prog/site_scons/builders/linux/sysroot/usr/include/stdio.h
-rw-r--r--# 1 user staff 29949 Sep 16 10:56 /Users/user/Development/com/prog/site_scons/builders/linux/sysroot/usr/include/stdio.h
This even happens when I use crosstool-ng and use their sysroot.
It's like clang doesn't look in the sysroot I set?