'getconf POSIX2_C_DEV' returns 200809 without c99 frontend to either clang or gcc.
More or less a scripting annoyance.
P27 of IEEE Posix-2008:
The system supports the C-Language Development Utilities option. The utilities in the C-Language Development Utilities option are used for the development of C-language applications, including compilation or translation of C source code and complex program generators for simple lexical tasks and processing of context-free grammars. The utilities listed below may be provided by a conforming system; however, any system claiming conformance to the C-Language Development Utilities option shall provide all of the utilities listed.
P435 of IEEE Posix-2008:
The implementation supports the C-Language Development Utilities option. If this symbol is defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by sysconf( ) shall either be -1 or 200809L.