Fix MacOS 10.15 Clang Link

Cannot build C program after upgrade to MacOS-10.15

Why

This is because apple change the path to the clang, those fundamental things are collected into /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk

Following these steps

• open terminal and install xcode developer tools by

xcode-select --install

• make link in /usr/local/include/

sudo ln -s /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/* /usr/local/include/

（Update） Install Ruby with rbenv

rbenv directly compile Ruby from source code. Therefore, we can still see some core files are missing in /usr. To install ruby, $(xcrun --show-sdk-path)/usr/include/ should be linked as /usr/include with csrutil disabled. about csrutil

The scripts below should be put into ~/.zshrc. Comment these lines out after ruby is installed or it might influnce original C/C++ environment.

# Setup Compiler paths for readline and openssl 
local READLINE_PATH=$(brew --prefix readline)
local OPENSSL_PATH=$(brew --prefix openssl)
export LDFLAGS="-L$READLINE_PATH/lib -L$OPENSSL_PATH/lib"
export CPPFLAGS="-I$READLINE_PATH/include -I$OPENSSL_PATH/include"
export PKG_CONFIG_PATH="$READLINE_PATH/lib/pkgconfig:$OPENSSL_PATH/lib/pkgconfig"

# Use the OpenSSL from Homebrew instead of ruby-build
# Note: the Homebrew version gets updated, the ruby-build version doesn't
export RUBY_CONFIGURE_OPTS="--with-openssl-dir=$OPENSSL_PATH"

# Place openssl@1.1 at the beginning of your PATH (preempt system libs)
export PATH=$OPENSSL_PATH/bin:$PATH

# Set SDKROOT
export SDKROOT="$(xcrun --show-sdk-path)"

ref

https://github.com/rbenv/rbenv/issues/1247#issuecomment-626241187 https://zhile.io/2018/09/26/macOS-10.14-install-sdk-headers.html