mail/astroid: fix build with GCC-based architectures

Use C11 compiler:
/usr/local/include/webkitgtk-4.0/webkit2/WebKitUserContentFilterStore.h:49: error: redefinition of typedef 'WebKitUserContentFilter'
/usr/local/include/webkitgtk-4.0/webkit2/WebKitUserContent.h:141: error: previous declaration of 'WebKitUserContentFilter' was here

Approved by: mentors (implicit approval)