pi@raspberrypi:~/Cacao $ swift build
Fetching https://github.com/PureSwift/Silica.git
Fetching https://github.com/PureSwift/Cairo.git
Fetching https://github.com/PureSwift/SDL.git
Fetching https://github.com/PureSwift/CSDL2.git
Fetching https://github.com/PureSwift/LittleCMS.git
Fetching https://github.com/PureSwift/CCairo.git
Fetching https://github.com/PureSwift/CLCMS.git
Fetching https://github.com/PureSwift/CFreeType.git
Fetching https://github.com/PureSwift/CFontConfig.git
Cloning https://github.com/PureSwift/Silica.git
Resolving https://github.com/PureSwift/Silica.git at 1.3.1
Cloning https://github.com/PureSwift/CFontConfig.git
Resolving https://github.com/PureSwift/CFontConfig.git at 1.0.1
Cloning https://github.com/PureSwift/CSDL2.git
Resolving https://github.com/PureSwift/CSDL2.git at 1.0.1
Cloning https://github.com/PureSwift/Cairo.git
Resolving https://github.com/PureSwift/Cairo.git at 1.2.3
Cloning https://github.com/PureSwift/LittleCMS.git
Resolving https://github.com/PureSwift/LittleCMS.git at 1.0.2
Cloning https://github.com/PureSwift/CLCMS.git
Resolving https://github.com/PureSwift/CLCMS.git at 1.0.0
Cloning https://github.com/PureSwift/CCairo.git
Resolving https://github.com/PureSwift/CCairo.git at 1.1.1
Cloning https://github.com/PureSwift/SDL.git
Resolving https://github.com/PureSwift/SDL.git at 1.0.1
Cloning https://github.com/PureSwift/CFreeType.git
Resolving https://github.com/PureSwift/CFreeType.git at 1.0.4
error: nonWhitelistedFlags("Non whitelisted flags found: [\"-D_REENTRANT\"] in pc file sdl2")
===================
PRETTY_NAME="Raspbian GNU/Linux 9 (stretch)"
NAME="Raspbian GNU/Linux"
VERSION_ID="9"
VERSION="9 (stretch)"
ID=raspbian
ID_LIKE=debian
HOME_URL="http://www.raspbian.org/"
SUPPORT_URL="http://www.raspbian.org/RaspbianForums"
BUG_REPORT_URL="http://www.raspbian.org/RaspbianBugs"
How to fix that?