macOS Installing pycrypto — Clang ‘stdio.h’ Not Found

While installing a Python package that builds a C extension with Clang, I kept seeing:

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clang-4.0: warning: argument unused during compilation: '-L/usr/local/lib' [-Wunused-command-line-argument]
src/_fastmath.c:29:10: fatal error: 'stdio.h' file not found
#include <stdio.h>
         ^~~~~~~~~
1 error generated.
error: command 'clang' failed with exit status 1

Checked Clang:

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clang --version
clang version 4.0.1 (tags/RELEASE_401/final)
Target: x86_64-apple-darwin18.5.0
InstalledDir: /usr/local/opt/llvm@4/bin

Looks fine… Tried setting env:

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export "CFLAGS=-I/usr/local/include -L/usr/local/lib"

No luck. From Stack Overflow, the fix was to install SDK headers:

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cd /Library/Developer/CommandLineTools/Packages
open macOS_SDK_headers_for_macOS_xx.pgk
# choose the pkg matching your macOS version

Run the installer UI, proceed… It works. Then pip install pycrypto succeeds.