clang: warning: no such sysroot directory: '-mmacosx-version-min=10.5' on compiling LLVM (3.9.0) - c++

Question

I am compiling LLVM (3.9.0) using CMake (3.6.2) on my Mac (OsX) using GCC, but somehow I am getting the following error with following GCC configuration
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.3.0 (clang-703.0.31)
Target: x86_64-apple-darwin15.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
[50%] Building C object projects/compiler-rt/lib/builtins/CMakeFiles/clang_rt.eprintf.dir/eprintf.c.o
clang: warning: no such sysroot directory: '-mmacosx-version-min=10.5'
/Users/Ritzy/llvm_src/llvm-3.9.0.src/projects/compiler-rt/lib/builtins/eprintf.c:14:10: fatal error: 'stdio.h' file not found
#include <stdio.h>
^
1 error generated.
make[2]: *** [projects/compiler-rt/lib/builtins/CMakeFiles/clang_rt.eprintf.dir/eprintf.c.o] Error 1
make[1]: *** [projects/compiler-rt/lib/builtins/CMakeFiles/clang_rt.eprintf.dir/all] Error 2
make: *** [all] Error 2
I am using following CMake command:
cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ -DCMAKE_INSTALL_PREFIX=$INSTALL_DIR ..
cmake --build .
I spent hours figuring out what was wrong. Even tried to using gcc6 and clang but in vain.

Answer 1

Here's an example of what the command line flags supplied to a MacOS/X clang invocation might look like:
-pipe -stdlib=libc++ -std=c++11 -stdlib=libc++ -O2 -std=gnu++11 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk -mmacosx-version-min=10.7 -Wall -W -fPIC $(DEFINES)
Note in particular this part:
-isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk -mmacosx-version-min=10.7
-isysroot tells the compiler where to look for header files, and then the next argument -mmacosx-version-min=10.7 tells it what minimum version of MacOS/X to maintain backwards compatibility with.
In your case, your CMake invocation probably isn't specifying a value for the -isysroot argument, instead it probably has something like this:
-isysroot -mmacosx-version-min=10.5
... where the path that was intended to be supplied after the -isysroot flag has been left blank (i.e. an empty string) for some reason, and that is why "-mmacosx-version-min=10.5" is being interpreted as a path to look for header files in (which of course will not work).
As to why the path argument was left blank, that is something you'll have to investigate on your own.