Put these into a directory (e.g. $HOME/Development/temp/ios), chmod +x build and ./build

build

clang -x c -arch arm64 -fmessage-length=0 -fdiagnostics-show-note-include-stack -fmacro-backtrace-limit=0 -std=gnu99 -fmodules -gmodules -fmodules-cache-path=$PWD/ModuleCache -fmodules-prune-interval=86400 -fmodules-prune-after=345600 -fbuild-session-file=$PWD/Session.modulevalidation -fmodules-validate-once-per-build-session -Wnon-modular-include-in-framework-module -Werror=non-modular-include-in-framework-module -Wno-trigraphs -fpascal-strings -O0 -fno-common -Wno-missing-field-initializers -Wno-missing-prototypes -Werror=return-type -Wdocumentation -Wunreachable-code -Werror=deprecated-objc-isa-usage -Werror=objc-root-class -Wno-missing-braces -Wparentheses -Wswitch -Wunused-function -Wno-unused-label -Wno-unused-parameter -Wunused-variable -Wunused-value -Wempty-body -Wconditional-uninitialized -Wno-unknown-pragmas -Wno-shadow -Wno-four-char-constants -Wno-conversion -Wconstant-conversion -Wint-conversion -Wbool-conversion -Wenum-conversion -Wshorten-64-to-32 -Wpointer-sign -Wno-newline-eof -DDEBUG=1 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.2.sdk -fstrict-aliasing -Wdeprecated-declarations -miphoneos-version-min=10.2 -g -Wno-sign-conversion -Winfinite-recursion -fembed-bitcode-marker -o test ./test.c

test.c

#include <string.h>
#include <stdlib.h>
#include <stdio.h>

int main()
{
  const char * s = "this is a test";

  long x = strlen( s );
  char *t = strdup( s );

  printf( "X=%ld, t=%s", x, t );

  free( t );
}

adding this:

#if __DARWIN_C_LEVEL < 200112L
#error "Test"
#endif