'stdio.h' File Not Found When Compiling Hello World - c

Question

Edit: added output of $cc -v main.c at the end of this post.
I'm using an old Xcode because my kernel extension will support El Capitan. That works with 8.3 by copying the 10.11 SDK from an older Xcode build.
#include <stdio.h> // 'stdio.h' file not found
int main( int argc, char **argv )
{
printf( "Hello World!\n" );
return 0;
}
$ ls -l /usr/include/stdio.h
-r--r--r-- 1 root wheel 19154 Feb 4 2017 /usr/include/stdio.h
$ ls -l /Applications/Xcode_8.3/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/usr/include/stdio.h
-r--r--r-- 7 mike staff 19154 Feb 3 2017 /Applications/Xcode_8.3/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/usr/include/stdio.h
I'm trying to build a userspace command line tool to test my kernel extension.
This is a moderately common problem but none of the solutions I've tried so far have worked. There are lots of reports of it happening in Visual Studio, not just Xcode.
Just now I reinstalled the Command Line Tools. I changed the SDK from "latest macOS" to "macOS 10.12".
The source file is called "main.c" - that is, not "main.cpp".
$ cc -v main.c
Apple LLVM version 8.1.0 (clang-802.0.42)
Target: x86_64-apple-darwin16.7.0
Thread model: posix
InstalledDir: /Applications/Xcode_8.3/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
"/Applications/Xcode_8.3/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang" -cc1 -triple x86_64-apple-macosx10.12.0 -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name main.c -mrelocation-model pic -pic-level 2 -mthread-model posix -mdisable-fp-elim -masm-verbose -munwind-tables -target-cpu penryn -target-linker-version 278.4 -v -dwarf-column-info -debugger-tuning=lldb -resource-dir /Applications/Xcode_8.3/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/8.1.0 -fdebug-compilation-dir /Users/mike/Projects/RCI/trunk/HD1/ButtonTest -ferror-limit 19 -fmessage-length 80 -stack-protector 1 -fblocks -fobjc-runtime=macosx-10.12.0 -fencode-extended-block-signature -fmax-type-align=16 -fdiagnostics-show-option -fcolor-diagnostics -o /var/folders/xc/jc2d96hx25l9ntd2vgnh124m0000gn/T/main-7206a2.o -x c main.c
clang -cc1 version 8.1.0 (clang-802.0.42) default target x86_64-apple-darwin16.7.0
ignoring nonexistent directory "/usr/local/include"
#include "..." search starts here:
#include <...> search starts here:
/Applications/Xcode_8.3/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/8.1.0/include
/Applications/Xcode_8.3/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
/usr/include
/System/Library/Frameworks (framework directory)
/Library/Frameworks (framework directory)
End of search list.
"/Applications/Xcode_8.3/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld" -demangle -lto_library
/Applications/Xcode_8.3/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/libLTO.dylib -no_deduplicate -dynamic -arch x86_64 -macosx_version_min 10.12.0 -o a.out
/var/folders/xc/jc2d96hx25l9ntd2vgnh124m0000gn/T/main-7206a2.o -lSystem
/Applications/Xcode_8.3/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/8.1.0/lib/darwin/libclang_rt.osx.a

Answer 1

I Should Not Drink And Code:
My project had at first just one target, for a kernel extension. Later I added a second target for a command-line tool.
When I created a new source file for my command-line tool that source somehow got added to the kernel extension's target. Changing the tool's header search path settings had no effect on my problem because the source was not in the tool's target.
Kernel Extensions - device drivers - have headers that are only from Kernel.framework, in which there is no stdio.h file.
I'll shut up now.